As much as I do like the game, there are a few things that are just incredibly annoying to me – pet-peeves if you will:

• Annoyance:  Contrived, geographical “pathing” – where the landscape has been forced into “walls” making you go where the game devs want you to go.  It really sucks to fall in a river that you can’t get out of until you swim ten miles up/down stream (Yay Chrace!).  AND places where folks obviously USED to be able to walk, but now there’s a big pile of blocky rocks in the way to keep you from using that convenient path.
• Annoyance:  Caves, or other confined areas with billions of mobs and that one “kill task” mob at the very, very back – where it takes over an hour just to fight your way back, hoping you don’t mass-aggro die (usually the mobs would be easy to kill 1-vs-1, but typically you proximity-aggro at least 3 at a time and they start respawning before you can get that 3rd one killed).  And if you ever get back to “the one mob to rule them all” and get it killed, you just “book out” or ‘cide just to save another hour’s frustration fighting your way out.
• BUG: Disappearing Pet Bars (and disappearing pets for that matter).  I’ve really only noticed this on my Squig Herder – you’re happily trotting along and for no apparent reason your pet-bar fades away. Or you notice your pet is missing, but the action bar still shows them active – grrrr.
• BUG: “Target Not Attackable” or “Not in Range” while attacking an enemy that seems to be right in front of you and it doesn’t appear to be having ANY problems smacking you around…

(EDIT 2008-09-27 – a few more peeves after playing some more]

• BUG: you can’t select a short character if they’re behind a large character from the character selection screen, AND there’s no way to re-order the characters, so my goblin Shaman will forever be lost behind my big, overgrown Chosen…
• Annoyance: 20 second delay between sending mail – this is something that should be addressed server side so the players don’t see it (like WoW, LOTRO, and just about everybody else that has in-game mail)
• BUG:  Pets/Turrets that can attack without regard to Line-Of-Sight, grrr – I hate getting shot through the ceiling/floor/walls, etc.

I think those are the main problems that really up the blood pressure and take me out of “immersion” from the gaming experience. Other than that, the only problems I have are “educational:” Players/Groups that don’t know what they’re doing. But that’s a whole ‘nother story….