It's not airtight, it just works well enough. I don't believe gameplay was the big factor why MW2 is more popular than other FPSes considering it doesn't play better and only has a pavlovian machine going for it, it sold on the strength of CoD4 (supposedly that had a very strong SP campaign too) and no major fuckups since then (except for dedicated servers, javelin glitch, OP dual shotguns, witchbladers, ... but except for the really annoying matchmaking system and the host migrations those aren't THAT major in their impact). It's really suspicious how many "yay, you rule!" praises the game vomits at you for every frag, sometimes I get modifiers for my frag popping up for like 10 seconds after the kill. It really seems to be all about making you feel like the hero, flying on a plane fragging everyone in sight, not really much about playing seriously. Just a casual game designed to make you feel good, not a tough game to challenge your abilities and especially not a team game that requires working together (not that random matchups ever work together, maybe that's how the game succeeds: Teamwork doesn't apply but a team always means someone else to blame for the loss).
I won't hold the fuckwits who complain about "campers" in a domination match against the game though.