Arena, AKA the implementation of PvP endgame.
Transmog would be the biggest feature I use, though.
LFR, and my signature explains why.
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Transmog, Achievements, account wide mounts and pets, CRZ
LFG > Account wide pets and achievements > Pet Battles > Daily Quests > Arena > Transmog > LFR > Reforging > CRZ
I think something like that. LFG was good as it made dungeons much less of a chore. It meant that you could do runs consecutively a lot easier and also meant that someone leaving half way through didn't force you to wait another half hour. I didn't mind the idea of forming a group in General or Trade chat, having to run to the instance, using the Meeting Stone to summon slackers, all of that was fine, but the implications of one person leaving (even for reasons out of their control) were just too much.
The rest of my choices were mostly based on what actually added to the game. Reforging was a quality of life improvement and a very useful one, especially if you play a class that has to work around haste points, but losing out on an item due to it's lack of hit is nothing compared to having an entirely extra thing to do in the game.
Arena I rated so far down just because it never really interested me too much. If my list was from a designer perspective, Arena would be much higher, as I imagine it contributes massively towards subscription numbers. I would also presume that less Arena players unsubscribe towards the end of a season, compared to Raiders that unsubscribe to wards the end of a Raiding tier, making Arena players a much more consistent source of income. With the same principals, in the mind of a developer, LFR would be much higher on the list too.
---------- Post added 2013-01-20 at 12:28 AM ----------
Not necessarily better than the other features already mentioned but I'd just like to give some praise to the Guild bank.
Before it was implemented managing mats for flasks, pots, enchants, food etc was a pain. Slowed down raid starting as we all would bring our mats and have our alchemists make our flasks in the raid. We had our raid leader have a bank alt solely for all the various mats and BoEs and keeping track was problematic. Needing to wait for raid leader to come online to get access to those items was also a problem. Many of our raiders would have an excess of mats but sharing them was difficult.
Now it's easier to share what we don't need, save what we will need and supply our crafters.
Also an honourable mention to the UI, both what is implemented in the default UI and what is available through the API.
Not sure if transmog, account wide mounts/achievements/pets or pet battles, i guess LFD was pretty great but i started playing right before it was implemented so didn't really see any difference.
LFR definitely. Even though I don't use it.
You have to look outside the box to understand the meaning of LFR. It means harder Normal and Heroic modes since now they don't have to really answer the "I'm a casual and I want to see the content" anymore.
Also it doesn't matter how hard the content is for the very very top end guilds(Top 10 world). They will clear the content in 1-2 weeks regardless. Even DPS check bosses can't hold them up really, people will figure out shit that will get them the kill.
This was seen on Ultraxion 25m HC, they used a Hunter pet to tank the boss and most recently on Gara'jal, where Mages spellstole a buff from the caster mobs inside the shadow realm to gain the necessary tiny bit DPS for the kill.
Worst thing ever. Those thing just kill realms community. Maybe you enjoy playing with people from others realms who don't give a fudge about you.. I don't.the Dungeon Finder/Raid Finder tool
They can't even says Hi..
After that I really appreciated how important the dungeon finder is to WoW, even if it did have negative consequences regarding realm communities.
EDIT: As for what I think is the best post-launch feature... I'd have to say probably the dungeon finder, that was what I missed most when I played SWTOR, which reminded me a lot of classic WoW. Also, DKs were a good addition, though I might be biased.
Account wide stuff worst
Transmogrification. Though I have equally enjoyed and been frustrated over the feature, for me it is still the best feature thus far. Having the option to customize your character's appearance to a greater degree beyond the creation screen. It also gave a whole new meaning to rewards. Such as you might work towards one reward for it's usefulness but now almost all available gear have another bonus, appearance, factor.
"Enjoy your power"