User interface probably needs a complete overhaul
So having really only played for a day and a half I can note some pretty glaring interface problems that make the game hard to enjoy, and would likely be an easy fix.
The quest/map system is pretty terrible.
For this I'd suggest doing something akin to how Skyrim handles quests. Namely in your journal/quest log maybe have the option to select which quests you want active. Once a quest is active the quest (!) (and perhaps should be a little bigger/easier to see) should always appear on your main map (i.e. you should not have to go into the quest log to open the map from there to see where to go). I'd also suggest maybe doing those glowing arrows that the guild leves provide which help dictate where to go.
For quests I'd also suggest that you make them easier to spot when you become close to them, and perhaps have a different map icon (instead of the !) if the quest has not been activated yet.
Also of course, this isn't so much interface but increasing the number of guild leves available (as in different things to do, not so much the number you can do a day) and quests in towns/different areas would help a ton. WoW had probably like 40+ quests per city. I think I've seen about 3 so far.
The lack of an AH is pretty game breaking almost.
I'm including this under interface because it's pretty essential and what selling interface we have so far is more than lackluster.
Now I think I heard they had plans to fix this but I can't believe that this wasn't part of the game yet. It's pretty much impossible to get gear for my level without taking up a craft. Going back to quests, perhaps including more quests that gave out decent gear would be a pretty great idea. But then you'd also need to have waaaay more quests.
Also the npc vendors seem to only sell level 1 gear. I feel like maybe vendors should scale to your level and offer more as you level up perhaps? Also given how the market works (or doesn't really), npc gear prices seem kinda high. Again this would be fixed with the addition of more quests offering more money though.
Double clicking a human player should open up the Check menu (or the /tell option) - Right clicking should enable multiple options like these to be selected.
And I should be able to buy from someones bazaar if I click on them even if they are not in the market area (if this is possible I haven't figured out how ;x)
User interface mod support
This probably would be hard, but including a customizable UI through mod support (ala WoW) would be so amazingly helpful.
Opening the main menu constantly
I don't like that I had to do it ;x I mean there are options that allow you to customize what does what but I think a default button for your journal and gear and attributes/etc should have been on the default. Just a small gripe I suppose.
Mouse usage seems clunky.
Like.. I really like using the mouse to move the camera around but selecting targets and such seems to really only work half the time ;x and it's hard to select things with the mouse in battle.
Attacking mobs
When in active attack mode, provided you aren't already targeting an enemy, if a mob attacks you - you should then auto target that mob. I seem to have to struggle through pressing tab and enter or clicking on the mob with the mouse when facing multiple enemies after killing the first one.
If in passive mode you probably obviously just want to run away so I feel switching to passive mode, while in mid battle, should deselect the current mob and allow you to flee easier.
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If I think of more stuff I'll add to this list, but these are noticeable mostly easy fixes that I think would help the game a lot. Harder fixes include the mod support and much more quests, but hopefully the latter is being worked on and the former considered.
My main gripes with the game having played so little seem to be the same as a lot of people.
Lack of things to do ---> fixed with more quests, more leves
Lack of a way to gear up ----> fixed with AH, more quests, better npc venders
Bulky & hard to use interface ----> hopefully fixed with the above as well as with other additions to combat to make it play smoother and I'm sure a lot of other players ideas.