* A real Auction House / Market system. With a good relational database and a requirement to filter BEFORE getting goods, performance should be fine.
Watch Quackecon presentation - they will add market NPCs. Guilds can rent them to sell their wares to public. Very nice workaround IMO

* Real ability to Group with friends in ALL the quest content at any time. Not being able to group with friends for content is just killing the social part of ESO.
You can for 99,99% Except for personal story. It would be kinda funny if Vestage came along with 5 mates saying I am the chosen one, hear of Tamriel and these are the guys that will hold my heroic hand.

Besides personal quests you can do it all in groups

* A real Wardrobe/Vanity armor system. What a waste of development time on all the great armor and gear looks that will never be seen.
Would you care to elaborate on this ? Like player made costumes?
Because as we speak any player ( provided he or she has skill and Motifs ) can make armor and weapons in any style

* More and accurate information on Spells, Skills, and other game systems. The current level of information is just pathetic.
What part of spell information confuses you ? Because you can not see spell X will do Y DMG against dummy. DPS as allways depends on too many factors to describe in a mere information window. You do get information of what the spell does and base damage for your character ( this is dynamic for every caracter )

* More feedback on-screen regarding short term effects both on self, on group members, and on targets. Currently, there is just no way to know what is going on, or why.
True, but it would need to be done very gently so not to break immersion. For those that do not mind, there are addons.

* Improved performance and responsiveness with combat systems, skills, and abilities. For instance right now, I have an 'instant' cast spell that requires a 1.5 second animation to finish BEFORE the spell is fired off. How is that different from a spell that has a 1.5 second cast time, and then fires? And, won't even get into Weapon Swapping issues.
This is a work in progress ( again watch Quackecon ). Right now you do cast instantly, but you can not spam it every 0 seconds untll you run out of magica. Working as intended.

Weapon swap has been improved and is being worked at ( Quackecon )

* It is time to address the rest of the combat related bugs. They have become less frequent, but are still there. Such as complete skill bar lockouts.
Bugs are there in every MMO in existance. Set up a bug report, put information on support forum, wait for solution.

* Loot itemization in ESO is really, really bad. Fixed rewards for quests? That is so 2004 design. I have not received a usable quest reward in something like 30 levels. Boss drops? Right. Mostly useless junk. Folks run dungeons once for a skill point and then never again. Chests? More useless vendor/store trash. That includes the Treasure Chests. Have not gotten anything but useless single-stat blues from them ever since the huge chest nerf.
Itemization is being looked at ( amazingly but it was adressed as an issue on Quackecon presentation. As it stands there will be changes in this regard. However player made loot will remain BiS

There is so much more in ESO that is underdeveloped and/or bugged. Additional content at the back end is not going to help ESO. A solid MMO game would do wonders for ESO.
Your "issues" have been adressed.

Every MMO out there is "underdeveloped" in regards you can never finish it. Thats we play MMOs, because there is no The End for game.

Phasing has been addressed as well.

Not fixed completely yet, but I do believe there was a patch that made some changes to phasing issues.

I do remember a interview with ZOS Paul Sage on Twitch ( forgot who was the host ) where he stated that phasing is getting looked at.

But like I said, pretty confident they did some changes to this allready.

a server wide auction house wont work. I've used mega server auction houses before. and I was usually out bid in 5 to 10 seconds. they have mentioned before that it wont work in eso. the guild kiosks are a step in the right direction though. but I agree with @phantax. they will be exploited somehow someway when they first come out. but I'm sure they will get it straightened out in an update or two

and I also would love a true vanity system.(seperate slots for armor styles only) but I believe they said at qcon that there wont be one, ever. if I'm not mistaken.