The mobile apps mess up #1 - one touch and the generic is sent, there's no pause or chance to edit; hate that. As for endorsements, it's the gamification of LI (see also, K). I made up a bogus category but really, I'm not approving any except from those who know for real. AND I'm considering opting out of them all together. 

My other 2 peeves go hand in hand; using social rather than being social. First, participating in groups and what not.. only to post your own links. Second, the random LI invite from total strangers w/ zero basis. Just b/c I want to network doesn't mean any and everyone. If we've never crossed paths, follow my Twitter, read my blog, etc. But if you're in a place we'll never meet, an industry I'd likely never work for, aren't looking for anything more than vanity numbers and/or people to spam as you look to get jobs or sell crap.. that is NOT networking, NOT social. FWIW.