1) Matchmaking/ Empty Lobbies.
As someone located in North America, it was frustrating to constantly be matched up against South Americans in CS:GO competitive when it first debuted. Same thing goes for being matched up with Russian players in MW3. Getting matched up with Japanese players was cool though. They had some pretty neat tactics I'd not considered before. No one's going to have fun when you've got a 200+ ping to each other. At the same time, at least it matched you up if there was no one else near you online. I guess I could make more friends? But it's easier to just blame the current system than to find fault with myself. :]
2) Crashes/ Bugs/ Glitches.
I guess? My experience hasn't been too bad. There was one time I Hook & Tackled myself under a staircase on Elven Ruins. (Zone door teleported me back out, luckily.) Almost as good as when I tried to Flank Attack, went shooting into the sky, fell back down and lost all my hp.
Then there's always the random trips to Narnia, but luckily those haven't happened in about a month or so.
3) Inability to remove/ swap grips between weapons.
Like you can in single-player. It's fairly low on the totem-pole compared to bugs/ crashes. But considering everything's RNG, it would be nice if when I finally get that cool new staff to replace my current one, I'd be able to swap out that sweet tier-3 grip because wtf heal-on-kill cloth. Or I could just recycle grips to get better materials, maybe. I'll probably just spend a couple hours doing that this weekend.