Gamma World is probably the best iteration of the D&D 4th edition rule system. It doesn't have D&D's massive list of powers, and it's quite essential. It has all you need to play and enjoy the game in a 160 pages book that even includes monsters and an adventure, as opposed to D&D giving you the same in three manuals of more than 300 pages each.
Weeding out the superfluous made a huge difference. In D&D, creating the character's build was actually more fun than playing it. Here you don't have to study that hard. Character creation is quick and quite fun. You have random origins that create weird and fun characters, but it's well designed, and no matter your luck, you'll end up with a functioning character very well merged into the setting.
I don't mind the Alpha Powers and Omega Tech cards either. They add fun and flavor (and some randomness of the fun type). Making them collectable was quite silly (really, did they expect players to buy booster packs for an RPG?) but you have all the fun you need in the starting box, which is, by the way, quite gorgeous, with maps and tokens as well as a sturdy, compact manual and a deck of cards.
Of course this is a traditional game. As well balanced and built as it is (making an encounter is an easy and fun job for the GM), it lacks several things that would make it really shine. For example, characters tend to be a bunch of powers and numbers with little in the way of having quirks and traits that build a story. The character sheet says what your character can do, but nothing about who he really is. Had they had traits like Burning Wheel's beliefs, instincts or goals, they would be easier to turn into real protagonists of a story (not that it would be difficult to implement them). But for a mainstram game, it's quite good and fun. I definitely recommend giving it a go. Besides, who wouldn't love to play a fungoid robot or a sentient cockroach swarm?