Well, due to an injury I'm a bit behind some of my painting and modelling projects. That said I thought I'd use the time I have recovering to discuss the units available, section by section, and how they have transitioned to 7th edition.
The Avatar of Khaine
The Avatar was a unit I used often since the release of the new codex. With the notable exception of the invulnerable save it pretty much just got a ton of buffs. Higher BS, more wounds, higher Initiative, fleet, more attacks, and access to a number of exarch powers.
Game Play - The Avatar is a key piece in a foot based force. I like to use mine to anchor foot lists. Placing it between 2 full size foot units of Dire Avengers with shimmer shield helps make the most of his fearless bubble, and he'll either draw fire that would otherwise go to more precious units or your opponents can ignore him at your peril. One thing I would have liked to see is for the wailing doom to be +2 str and have armourbane in CC as it would have been thematic to the shooting version of the weapon.
In 7th: The changes to smash took some of the sheen off the Avatar for me as it really limits what I'm willing to throw him against, but he's still a beast overall, and with Fast Shot he's worth taking, but you really need to design a build around him from the beginning. Aggressive play is key to getting the most out of the Avatar so have a plan to use him at the start of the game and hold true to it. Do not be disturbed when he takes a couple wounds and worry about protecting him. Throw him forward and force the enemy to respond to his threat and pressure.
Initially billed as the Eldar take on Batman in 4th edition the Autarch lost some stock with me in the new codex. Not much has changed for the Autarch itself, however some of the wargear available, such as mandiblasters, substantially changed, thus altering, and in many ways reducing the Autarch's usefulness. The requirement for the Autarch to replace a weapon to get a Remnant of Glory isn't a big deal, but the fact that he only comes with a shuriken pistol is. It means if you want the bonus attack for having two close combat weapons you must, at the very least, add a 3 pt tax on to your model. 3 points might not seem like much, but it can make adding the Autarch in to your list a real pain in the but for rounding out your army totals. In the prior codex the Autarch had a rule that allowed him to benefit from unit rules outside of infiltrate. The loss of that rule was also a loss of synergy for the Autarch as his inclusion is a negative for Swooping Hawks and Howling Banshees, and that's a shame really.
Game Play - The build you choose can really define the roll the Autarch plays. For instance I really like giving an Autarch a banshee mask and shard of anaris. the banshee mask paired with plasma grenades means you're all but guaranteed to go first on the charge. Add in the shard and you have a good chance to ruin the day of any character through a challenge.
In 7th - The Autarch has real appeal for me in 7th, in part because the bonus to reserves makes the null deploy list that much more potent, though it requires careful deployment and planning for the units that start on the table as they have to survive.
As an Ulthwe player saying the Farseer was my go to choice was a bit of an understatement. I'd often run two. The new codex not only boosted their mastery level from a possible two to three, it also made the farseer dirt cheap. The points I invested in farseers essentially dropped in half, though part of that was that runes of warding and runes of witnessing were nerfed to garbage. If they were 5 pts each I think they'd be more reasonable, or if they weren't one use only they'd be really tempting at their current cost.
Game Play - Farseers are a premier support unit for the Eldar and their use can really change quite a bit based upon what powers you get. The ability to alter your gamestyle to get the most used out of whatever combination the farseer gets is paramount if you play very different lists from game to game. If you're playing the same list all the time you're most likely hoping for the same spells as often as possible. I wish they'd give Farseers a second base attack, but alas, GW seems deaf to my longing here.
In 7th: The changes to the psychic phase has pulled some of the rug out from under Ulthwe armies. In the past I'd all but guarantee three powers per Farseer. Now I have to be much more selective. When I first heard about the new psychic phase I was fairly unhappy because I saw how it would limit my force. With only a single D6 getting added on to the total WC I figured it would scale really poorly, and I still feel this is the case. That said, it means I need to be much more strategic with the powers I choose to cast and can lead to a cat and mouse scenario when facing down an opponent. 7th has also opened up Daemonology powers, of which I've found Santic works very well for Eldar. Not to mention the option to get a bonus power through Psychic Focus. I don't use Farseers as much now, but when I do I back them up much more at the army building stage and I keep each one to a single discipline, which means I'll have as many powers as possible to choose from and keep myself flexible, and it means if I get a power I won't cast it feels less damaging to my overall selection of powers. I feel I should also make a special note here about the Runes of Fate, as only Farseers can take them. the Runes of Fate were harsh to have so many powers at WC 2 in 6th, but in 7th, because I can get additional psykers to act as batteries for warp charge it makes having a Farseer dedicated to Runes of Battle less problematic. However, Death Mission, which was the worst power in the game in 6th, has actually become worse in 7th. With the addition of a new phase to the game it means there are now 8 rolls between castings, which means with average rolls you will only have a CHANCE of casting it a second time (ie living through a single turn) 1/3 of the times you cast it. I don't find the benefits in any way meet the ludicrous costs of the spell. Another special mention is the GhostHelm, which in 7th only ignores a wound caused by perils... and until a FAQ hits to say it fully ignores perils it will have some wonky interaction (lose a power, but not a wound, etc).
The new kid on the block is a step between warlock and farseer. It comes stock with a witch staff (which I would have liked to see as an option for Farseers), but is mostly a warlock with plus one wound and leadership. It also has the spirit mark ability that on first read seemed like a throw away ability tossed on to the model, but I've used it to such good effect (and surprisingly don't often forget it) that I really respect it now. It doesn't require LoS, or a roll to hit, or a leadership test to work. It's a really cool passive ability for those of us that tend to roll 1's when shooting wraithcannons.
Game Play - The spirit seer is the only way to get conceal into a Wraithguard squad. It also has access to telepathy, so most folks will probably hope for invisibility. Note that if you go full Runes of Battle it means the Spirit Seer has an impressive 6 powers to choose from. If you're using a spirit seer you're probably running Iyanden, so there's a good chance you're giving up conceal for twilight voice, which does not have a mirror power, but is impressive in a wraith list.
In 7th - Like Farseers they have accesss to Daemonology, which once again means access to Santic. Sactuary can give a Wraithblade unit with axes a 3++, which makes for a seriously tough unit.
While not technically an HQ I'm including Warlocks. They are the backbone of an Ulthwe force. They are the same cost or slightly cheaper than in the prior book, depending on the power you purchased and with random powers they're not as dependable as before. The randomization of powers has really altered the unit's dependability and they are no longer a separate unit and a unit upgrade. They now work similarly to the 3rd edition book, being broken off from the squad and attached to another unit. They also lost access to a wave serpent, which makes them extremely vulnerable at the start of the game.
Game Play - I don't have any experience with a squad of bike mounted warlocks, and on foot they are capable, but the unit requires Farseers to back them up and make them truly formidable. Without that synergy they are a point sink. Knowing that it means they must be used aggressively. A unit of three Farseers and ten warlocks are approaching the point totals of a titan, so they have to be used just as aggressively. force your opponent to deal with them.
In 7th - There is some win/lose for Warlocks. For instance they can't attempt to cast the same spell more than once in a unit. I used to love having three warlocks with destructor. At the start of 6th that took a hit because it could suddenly be denied. Then with the new codex it took another hit because it became random. Occasionally I'd end up with more copies of destructor, but most of the time less. In 7th it's now flat out not allowed and is a bit sad. I don't think I've cast destructor a single time since I've moved to Australia. Then 7th disallowed units in a transport to cast powers on themselves, so they are forced to stay on foot. That said, they have access to Santic, so once again that +1 to invul save is a possibility, but more important to them (considering invisibility can be an adequate replacement for Sanctuary + Fortune) is the possibility of Infinity Gate. Deep Striking the unit makes them a terror for any army to deal with once they are tooled up (Gate of infinity plus prescience or guide on a unit all armed with singing spears is absolute nightmare fuel). Powers like Jinx or Misfortune also make opponents quake once they've been on the receiving end of a full on council charge. With the restriction to one attempt per unit it means you will most likely have many redundant powers in your warlock squad, which ultimately makes the other members important because they'll be warp charge batteries for your most potent spells.
Without covering Special Characters, the Eldar have a rather strong selection of HQ choices. Most of them are fairly utilitarian, but give you access to force multipliers.
My two favorite builds centered around generic HQs are as follows:
1) 2 CAD including 3 farseers, 1 unit of 10 warlocks, 1 unit of 3 warlocks, Hemlock. Disseminate 3 warlocks among guardian squads and when all your units are on the table you've got 23 WC. Most of my lists are designed to own one aspect of the game and otherwise be as flexible as possible and this build is designed to own, as much as Eldar can, the psychic phase. Some armies can still dominate this build, but they can't casually do it, they need to build their force from the ground up to do so.
2) 1 CAD including 2 full size units of Dire Avengers with shimmer shield Exarchs and an Avatar. A Farseer can be added for taste in this build (fortune can make that 5++ extremely potent), but ultimately I use it for position control. Keep marching forward and take the ground you need.
I hope people found this enjoyable and maybe informative.