I hate huge headers, big blocks of ads, "Latest topic" stats, shout boxes, etc all appearing before the actual forums. If you need to scroll down to see your forum categories, you're irritating far more than just me.
I hate installs of vBadvanced. Well, okay, let me clarify a bit.. vBa can actually be used well, but most people don't. Most people make a portal page with two sidebar blocks, a huge welcome message, one or two six - twelve month old news postings, and absolutely no link to their forums except for in a block in the left column..
So you get to their "portal" page and see a basic, pointless install of vBa. Now people who take time with vBa, and actually make a cms or a portal where it makes sense while still offering a decent UI, I don't mind at all.
I hate gaudiness.. How many sites actually need an arcade system? How many sites need vB's default calendar / FAQ? How many sites need to display hundreds of different things in the users "postbit" (that's what profiles are for), etc etc. I hate bloated features, I hate limitations (ie: you must post 5000000 times in order to get private messaging), I hate when admins make a sticky thread in every forum for "rules"..
And last but not least, I hate anal moderators and admins.