What works differently here is how those roles are played out. In ET: QW you're not simply jostling over spawn points, but working towards a central, map-specific objective. To complete this objective and win the map, you will have to complete other, contributing objectives, and each map also throws in secondary targets which, while they might not have a direct impact on the main objectives, will make the map easier as a whole. The clever thing is that a computer controlled commander is continually analysing these objectives as the game progresses, and offering them to the players according to their chosen class as missions. You can normally choose from one of several, and they're worth completing as there's always something in it for you.
You see, completing these missions gives you experience points (XP) which enhance your skills and abilities. Some of these are specific to the class you're playing, while others transfer between classes should you switch. What's more, XP, rather than kills, determines your placing in the final end-of-mission rankings, meaning there's prestige in playing your part to the full. Finally, each map is only one third of a longer campaign, so earning XP in the first two maps will make your time easier in the third. It's not a persistent experience system in the way that Battlefield's is, though the game does throw in a system of persistent rankings and medals. I think this strikes a nice balance between rewarding the hardcore player base and ensuring that casual players and noobs aren't instantly overwhelmed by super-powered killjoys the minute they hit a new server.
The upshot of this is that ET: QW is a game where everybody, from the hardcore player to the keen noob is at least vaguely aware of their role, and usually making some attempt to fill it. The grunts are racing forward to blow up objectives, capture spawn points or defend key areas; the medics are trawling the battlefield for troops to revive; the field-ops are deploying artillery and clearing the way with strategic air-strikes. There are still frustrating moments - why is that Strogg Technician wading into battle when he should be getting me back on my feet? - but overall the game has that feeling of teamwork and comradery that the Battlefield games so often lack.
Nor is that all that's clever. Linking the maps into campaigns does a great job of making you feel like you're playing a key part in the defence/destruction of humanity, while also encouraging players to stick to the server for a long-haul rather than jump ship every time they lose a game. Vehicles, too, are handled brilliantly, with the game defaulting to a friendly handling mode that means even novice pilots can take aircraft out and do some damage to the opposition, rather than themselves. Develop confidence, and you can then switch to an advanced mode for tighter control. The weapons offer an interesting and well-balanced arsenal, and the deployables and air-strike capabilities make for some spectacular death-ray carnage on the battlefield.