I don't consider 3E/3.5 to be rules heavy. Sure there are a lot of components, but overall it's not hard to get down. Now, depending on how much a GM adds on to the CR's it can get hefty. However, no one says that you need to add on any supplemental material, hence the term supplemental. IMO, 3E/3.5 is rules moderate.
That said, there are other games that take less time to get the rules down. Storyteller, FUDGE, FUZION, Feng Shui, and Savage Worlds are examples of these. The each take less time to learn and get to playing than D&D.
On the heavy end of the spectrum would be Rolemaster and (maybe) HERO (from accounts of my friends, I haven't acutally played it myself). They take more to learn whether from more base material or using a more complex dice mechanic.
For what it's worth, I'll take the 3E/D20 system over either end of the spectrum. I don't want a complex system, but I want one that will have a good, concrete set of guidelines to direct my rulings in play.
Kane