So many things I could put here. But, I guess, at number 1, I'd go with bad console porting, with everything that entails.
Nothing chaffs my crack like bad UI's, quick saves, sucky controls, or controls that cater to controllers instead of keyboard/mice, and very little control over video and sound options.
If given a chance to list 2, then my second least favorite thing would be open world pvp, in any mmo. I'm not particularly fond of pvp at all. But, I will every once in awhile jump in a battleground/pvp instance/whatever it's called in the current game I'm playing. But, to be forced into pvp when all I want to do is go out and complete a few quests, is so incredibly not fun, I just can't figure out why other people love to deal with it. What makes it worse, is I generally level up slowly, taking my time. So, I"m pretty much cannon fodder to all the people that have to race to end game, and then find the only thing left for them to do is go and make life miserable to anyone left behind.
I try to stay away from games with open pvp. There have been a few that I've played in spite of it having open world pvp (lineage 2 and aion come to mind), but those are few and far between.