I take it you've never played any edition of D&D then.
Again, the rules are actually a streamlined version of 3.5, with a few of the later things (such as the equivalent of actions points, for example) put into the system from the get-go.The rules are very convoluted and intimidating, and there are so many of them! Its more meticulously fleshed-out than GURPS, and often goes into far too much detail. It doesn't just seem clunky, it seems unplayable.