Well, when we switched to a rules-light system for about three or four months, everyone was having a lot of fun. We found that the d6 system's lighter system really encouraged players to take actions that they wouldn't try in D&D (not because they couldn't, but because they weren't sure how the rules would handle things). So, we had a play jump from one airborne car to another, stick a wad of plastic explosive on the windshield, and then leap into the air, to be picked up by the first flying car. We had the same character involved in a running knife fight (he threw knives) with two security guards with rifles - and the character was doing things like tumbling over computer terminals, rolling underneath doors, and jumping whole flights of stairs.
Yes, you can do it in, say, d20 Modern, but often the players think of the mechanics ("I can move thirty feet per round, or double that if I don't take any other action.... I don't think I could really make the necessary jump check rolls to clear that flight of stairs...")
I think Rules-Lite systems really encourates creativity on the part of the players, because you don't have feats and skills in front of you, telling you precisely what your character can and cannot do.
We stopped playing d6 because... well, D&D was getting to be fun again. I'm sure, though, that the next time we're all annoyed with the grappling rules, we'll pick up our d6's and go at it.