I think in total D&D has gotten more complex over the various editions. The prime factor for this is player saying
I want to make the character that I see in my head.A good test of this is Fritz Leiber's Gray Mouser who has a bit of fighting, a bit of magic-user, and whole lot of thief.
Back in the day this is the prime reason why people went to Runequest, Hero, GURPS and other FRPGs. The solution adopted was generally the list of skills with the players being allowed to pick which ones his character is good at.
However the list of skills is not universally appealing or simple in the way the straightforward class & level system of D&D is. In 3.X the designer came up with a pretty elegant way of allowing players to customize their characters yet retain much of the advantages of level and class. Also skill based system developed packages, or templates which took the list of skills and distilled into a specific set of options understandable to the average player. The two approaches learned from each other.
As for D&D this one of the main factors that propelled it into a smash hit. It also helped effectively suck the wind out of 2nd and 3rd tier RPGs by closing the biggest reason for changing away from D&D.
But coupled with the open ended list of feats, diversity of classes, and other things then we start running into the complaints about 3.5.