They use a bunch of gross old fruit as garnish
Sure, it took you a really long time to slice a couple lemons into wedges then open that jar of maraschino cherries. But maybe it's time to toss them out and let the fruit flies have 'em. I mean, they're already all over the container anyway.
They scoop ice with the glass… or their hands
The latter is wildly unsanitary, the former could result in shards of broken and chipped glass hiding invisibly in an ice tray, which could lead to some internal bleeding, which is also wildly unsanitary.
They keep pouring drinks out of pitchers
Sure, punches are super-hot right now. A pitcher full of Long Islands that's been sitting there for a week? No. Just no.
They constantly brag about their bartending credentials
Congratulations on completing a free online bartending course! And for supporting your local frame store so you could proudly display your "degree" alongside your status as a preacher of the Universal Church of Life. Thing is… that bartending course was about as legit as the seminary that allowed you to moderate your friend's wedding.