Each class takes some kind of learning to get adjusted to them. But there's also the matter of each one having unlockable weapons that give most of them different play styles.
For example, a Sniper wielding a rifle will want to stay away from enemy fire and just pick them off from a distance. Whereas a Sniper wielding The Huntsman (bow and arrow) will want to join his team in close distance shots as The Huntsman isn't really suitable for long range but is more useful for middle range shots.
Is there a difference between this and other FPS'? Well for starters, the game doesn't take itself seriously. Rather than taking on the role of being another ultra serious FPS, the characters all have a cartoonish look, with their own glowing personalities and phrases. The game features Capture the Flag, Capture Point, Territorial Control, Payload, Arena and King of the Hill. There's a crap ton of maps and one of the best things about TF2 is that the community can also make their own fully functioning maps, which also have a chance at becoming official maps.
Plus with the different classes there are, you won't necessarily feel restricted to just kill, kill, kill.
As for system requirements, these are the minimum requirements:
1.7 GHz Processor, 512MB RAM, DirectX® 8.1 level Graphics Card
Just for good measure, here's a good look at me playing as a Spy and Sniper (wielding The Huntsman) for a good idea on their playstyles:
http://www.youtube.com/watch?v=ZdKNSUMlQRw