Frequency of Play definitely matters. I DM'd a group from about 12th level to about 20th level, and we only played once every six weeks or so. Did it take four hours to cook up a CR: 19 NPC? Yes. Did I care? No. I had the time. Further, since we didn't play often, our sessions were fairly fast and loose. We didn't have time to get bogged down over minutia. If we'd played more often, I would have been less happy about the time it took to stat up NPC's, and I would have paid more attention to a few of the more fiddly rules.
Party Mix is one I haven't seen addressed. The group above included six PC's: a Fighter, a Ranger (TWF style), a Rogue, a Paladin, with no multiclassing or PrC's, a Cleric with four levels of Fighter, and a Sorcerer with levels of Mage of the Arcane Order.
Obviously, this was a melee-heavy, magic light crowd. Do fighters suck in 3.X? Not in this campaign. Do Wizards pwn? I dunno. CoDzilla? not with four levels of Fighter, no.
Splatbooks Goes without saying, bears repeating. 3.0 with just the core rules is obviously going to look and play very differently from 3.5 with Bo9S, Complete Arcane, MIC, etc. etc. etc.