I can understand the frustrations of DM's who don't have the time or ability to set up complex encounters and such. I hope that D&D 4e helps them enjoy the DMing aspects of the game more.
Like the OP, I have never had any problems running combats involving many varied kinds of critters. Just last weekend it was:
aasimar monk 13
pixie druid 9 with dire hedgehog animal companion
human ranger 7/beloved of Valarian 6 with celestial charger unicorn companion
elf paragon 3/wizard 3/abjurant champion 5/eldritch knight 2
human ranger 9/psion 3
vs.
half-fiend aboleth wizard 10
fiendish frogemeth
6 fiendish advanced slime chuul
12 fiendish advanced skum
6 fiendish advanced aballin
6 human slaves (who existed to frustrate the characters through flanking and by making zombie-like professions of love for their tentacled overlord)
This was a pretty nasty encounter set on the banks of an underground river that had been polluted by a gate to Juiblex's plane. My head didn't explode. The prep for the encounter took maybe 2 hours. I used d20srd.org and digital copies of WotC books in order to copy and paste monster statistics into Word and then modify them by template or whatever. (I only buy digital books these days; my world is too cluttered for anything else.) The most time consuming part of prep was printing off and laminating counters for the game, but I farmed this job off to my girlfriend and her son. Heh. (By the way, Counter Collection Digital v2.0 Gold is one of the best gaming purchases I've ever made.) The encounter took three hours to run and was very exciting to me and my players.
I have a full time career teaching English, and I have a family. I do have a bit of leisure time in the mornings and evenings, where I do most of my work--maybe a half hour a day. I have been playing D&D in some iteration or another for 27 years, but I can't say I have a huge amount of experience with v3.5. I've just gotten into the updated rules within the last couple years after a long time off from running 3.0.
Eh, I guess this post is mostly pointless. The new edition is coming, and there's nothing in this world or any other I can do to stop it. I understand what WotC is trying to do with the game. They're trying to make setup and gameplay as quick and painless as possible in order to compete with a world of leisure activities that includes things that are a lot easier to enjoy (out of the box) than v3.5 of D&D, like movies and video games and lovemaking. Since I don't have the same kinds of problems as some DMs, I don't really see the need for these changes, although I won't begrudge those who do.
I will say that some of the changes don't seem to have this goal in mind and really only exist (in my opinion) to shake things up. For example, while I know that multiple attacks and AOO and grappling can cause headaches, I don't seem to remember a lot of threads here or elsewhere complaining about erinyes, or the boring nature of the elemental planes, or the problem of referring to elves, dwarves, and such as races, or how cool it would be if wizards had to carry around magic balls. (And you thought the Pokemount threads got hot!)
Interestingly, I have just planned an adventure set in the elemental plane of water, with a jaunt to the City of Glass and a marid's palace and involving the ambitions of a kraken, and the place didn't seem at all boring to me....
Game on!