I've been running games since the early 80's and playing with most of these guys on and off since then so I'm pretty up to speed on how to approach a campaign for them. The issue was when I had 10-12 foes on the table and they all had feats, buffs, spells, round by round conditions, that had to be managed. So yes it was information overload in that regard, when I was spending a lot of time putting together spreadsheets to manage the combats I realized there had to be a better way. It was a great spreadsheet though.
Half my table could handle 3.x fine, the other half had to be reminded constantly about all the advanced abilities and applications of those. Which was a drag. But my guys are fairly casual, only a couple read books and work on stuff away from the table, for the others game night is the only time they are doing D&D. So we gave 3.0 a try and it collapsed under its weight around 10th level and we switched back to 1e. When 3.5 came out I figured...lets try it again! And it collapsed under its weight at the same point. Also buying a book that was just to explain all the rules in other books made me think, "Flexor...why are you doing this to yourself?"
IME 3.x was great for those who loved tweaking builds and all that, which just isn't my group outside 2 players. With a die hard group who were systems masters I'm sure it ran a lot better. But my beer, whiskey, and pretzels group is another story.
Now I'm pushing them through the meat grinder in Swords and Wizardry and having a blast. But I like to game like its 1975 so its all a great fit.