I would disagree that earlier editions didn't have a CR system. It wasn't coded the same way or called the same thing, but most GM's knew what threat level of an encounter was before they ran it. They knew when it was an easy encounter, they knew when they were about to break the party with a monster.
I don't think most of the information is optional. If a player casts a spell that requires a reflex save, the save number is not optional. Feats are a big part of 3E. If someone has improved grapple, you better know the stats for the monster. You can't play this game as a 2E game, nor would I want to.
In 2E a good GM codified the situation on the fly. That doesn't necessarily mean unfair to the players. A lot of situtation are much harsher on the players in 3E then they were when I ruled on them in 2E or 1E. This isn't a rant about fairness or balance. It isn't even about complexity. The save numbers aren't complex. The idea of monsters having stats isn't complex or even new.
It's more about the level of information the DM MUST have on hand to run a fair encounter and the amount of extra time it takes to create the game now.
Imagine running a mage or priest in combat as the GM now. Every spell can use a different save, so you have to have that information handy. 2e the players had a magic save that they kept handy. So instead of saying "roll a save take 15 or 8" and then moving on to the next issue of combat, I have to refer back to the notes to see which save Sniloc Snow Ball uses, then look at the mages sheet to get the DC of the spell, then tell them 15 or 8.
Say now the fighter now runs over and tries to grapple said mage and succeeds. Now the casting involves concentration checks, grapple checks, attacks of opportunity etc. This time adds up over the course of a long game and adds to the wear and tear on the GM. It makes creating the NPCs longer, it takes more time to prepare in general and the combats are far more complex from the GM's point of view.