Yes, 3e at first blush seems less deadly than AD&D, but critical hits and more deadly monsters belies that very quickly. Since all monsters have ability scores now, their attacks, damage, saves, AC, and hit points can vary wildly, even at low CR's.
Like a lot of DM's, one of the first 3e DM's I played under would decide that, if a monster has treasure they can use, they will. Since he was unclear about treasure distribution, he'd roll up treasure randomly, and eyeball anything that went beyond WBL guidelines.
We were fighting hobgoblins, and he'd determined their captain would have access to banded mail +1. Following the guidelines, he gave the captain 2 levels of Fighter, making him a CR 2.5. Finding that to be a little odd, he gave him another level of Warrior, making him a full CR 3.
Now by the rules, this changed his base stats from 13, 12, 11, 10, 9, 8 to 15, 14, 13, 12, 10, 8 (before taking his +2 Dex, +2 Con into account). Now that he was 4 HD, he got a +1 to an ability score as well.
When all was said in done, Captain Bloodaxe became a legend for years to come, with Str 16, Dex 12, Con 16, Int 13, Wis 8, Cha 12, 19 AC (Banded Mail +1, +1 Dex bonus, +2 shield), and 32 hit points!
We eventually pried that armor off of him only because of his horrible Will save (even bolstered by Iron Will it was only a +1), and we learned a valuable lesson about the monster creation rules- always compare the final result to an existing monster. In every respect save damage, Bloodaxe was superior to an Ogre (and I've seen more than my fair share of low level players taken out by a lucky Ogre crit, lol).
For most characters, "build" optimization really only started to matter by levels above 7-8, other than some Fighter builds, that usually still had a glaring weakness (like chain tripping vs. ranged attackers or big strong things with multiple legs- I watched a chain tripper completely fail to deal with a stock centaur!).
Though the right spells could be troublesome at any level, since the 3e team totally dropped the ball, thinking everyone was going to be using damage spells. Damage spells that were basically doing the same damage they'd been since the 70's, despite the fact monster hit points had dramatically increased.
When you launched a fireball into a crowd of foes and they were all still alive afterwards, you start looking at other options, and this is when people really started to see what things like web, stinking cloud, and sleet storm could do to a combat.
Having played with (and DM'd for) groups that think "optimization" is taking 2 levels of Fighter for bonus Feats, I will tell you that high level play in 3e was a miserable experience if you didn't have much system mastery. As a DM, I'd usually refuse to use spellcasters because I knew my players couldn't handle threats that did anything more dramatic than deal hit point damage.
Where an AD&D Fighter could always count on having decent saves, this was completely not true, and a lousy Hold Person was effectively a death sentence for a lot of characters.
