I'll go for my biggest pet peeve, forget my small gripes!
My b i g g e s t SL pet peeve might be more of a specific pet peeve, so everyone may not be able to relate to this exact situation, but I'm sure if you think about it you can conjure up something similar.
People, who ask me "Why not get a mesh body and bento head?" When I say, Nah I don't want one, they proceed with the question, "Can't you afford it?" or something else that is equally as insulting, seemingly without them realizing it. When I assure them that, Yes I can afford it, but no I still don't want one we begin an ouroboros circle. "But why not?" "SL would be so much more fun with one!" "You should at least look into it!" Sometimes followed by several brand name drops of popular Head and Body makers that I should certainly go visit the stores of! During the short time I've been active I'll say this has happened three, maybe four times and once, humiliatingly enough, with my former landlord. This landlord, after she took my rent, tried to badger me into getting a "normal avatar" and telling me I'd be much happier. I'd like to think I'm a nice, relaxed person so each time I bit my tongue and tried to get them to understand that I'm not interested means just that, and no amount of pestering will change that. I still don't think two of them got it and I left my rental as soon as my time was up.
So, I guess in tl;dr format of it is, my pet peeve on SL is when others try to tell you how to live your Second Life and what will enhance your experience as if they know myself better than I do.