I'm usually too baked to care, or even notice what everyone's bringing to the party. The host is my first, and only serious concern - consumables equipped? If yes, we're probably good to go as long as competence is detected in the loadout. Beyond that I don't really look very hard. I'm focusing more on whether I might switch to a character with more potential synergy.
I've hardly ever left a lobby simply because of what characters or weapons people are using, unless I happen to find 3 Infiltrators when I'm not playing as one. But even then, I'm more likely to just switch to an Infiltrator myself. When I say I don't care what other people do, I really mean it, but I'm not taking that kind of aggro dump if I'm playing something with weak shields.
As others have said, suicidal map combos on PS3 are a fairly obvious no-no, and I will always leave a preset Giant lobby because, well, you all know why. But let's face it, I'm probably gonna land in the game halfway through wave 1 anyway, right? Hydra's the only map with a warning light at this stage, although Rio will still make me very slightly wary.
The only lobbies I regularly leave are the ones where the host with the N7 rank in double figures, maybe playing a level 8 HV with no consumables readies up the moment I arrive. While I usually have a chuckle and just leave, I must confess I've been known to ready up too, and then just quit the game. Good luck out there, Soldier! If you're any good you'll still get the wave 3 credits.
Honestly, the occasional lengthy wait to fill the third and fourth slots has seen me readying-up with any level 20 player with a functional loadout on several occasions recently. Mostly good games too, and none of them have been with the Usual Suspect characters. It's not all doom and gloom out there. And what's with all the Vorcha? Have I missed a meeting? Pug Land is Vorcha Land on PS3 lately. GRAAAAAAGGGHHHHH!!! BIG MACHINE!!!!!! Makes me think of you every time PAT, even if you're on hiatus. VORCHA MASTER BAITS must return one day, if only to shout, "I'M ALIIIIVE!!!" one last time.
As for Ghost, it's probably my favourite map. Not for any particular reason, and it's not that I play it particularly well or have any wonderful guaranteed win strategies or anything - I just like it, and find Pug behaviour there can be unpredictable enough to pose an enjoyable challenge. It also amuses me now, remembering that I found it quite hard to 'work out' in SP. It can be a pretty big challenge as a melee Krogan if you happen to find yourself with a sniping squad of snivelling pyjaks. On the other hand, I had one of my favourite ever matches there with QMR - that Krogan Sentinel with Claymore and QMI with Indra (Edit: Apologies, I meant Valiant - I always mix them up) were hunkered down in the LZ for a hack. There were two dead pugs somewhere, so the hack was taking its sweet time, but it was glorious. Headshot! Hahahaha! We coulda held 'em off for days. 
These ramblings have been brought to you courtesy of Stoners International. Apologies to all those affected. Dammit Sinful, I coulda been playing for the last hour if I hadn't started reading this thread!