I too have been GMing for about 20 years. I have GMed dozens of systems, large freeforms; all sorts of stuff. Still, with d20 I have found that I get more nervous than I used to.
Keeping on top of everything is much harder, and requires a lot more skill than it used to. Dealing with combinations of bad guys, with differing skills/feats/abilities and making them all work effectively is hard. Add the element of trying to sort out if they should be using special attacks like Bulls Rush, Disarm, Trip or Sunder, and you have a lot to think about in a short period of time.
Then you have to think about the repercussions of the PCs using all of this, and combining it with powerful spells and magic items. d20 is a lot harder to run than anything else.
Also, because of the more options with skills and feats, I have found that players tend to rely on those for problem solving instead of role-playing. This is easy to deal with, but it sometimes upsets the players that you don't let a simple die roll do it for them; and that adds to the "do they hate what I am running" element.
I regularly ask my players for feedback; and ask them to make suggestions for what would work better for them. That works.
Also I have been lucky with my players, I have recently had them start correcting me when I do things that appear silly with monsters, telling me that there is a much better option that they can see. Sure, sometimes they don't have all the facts and I tell them the action stands as it is, but sometimes they are right, and I thwomp them with their own ideas.
Richard Canning