Maybe it's just my playstyle, but I literally had 0 trouble taking some powerful undead creature and reskinning them to be a "Human Necromancer Villain." I didn't feel constrained or limited at all. I felt like I could invest the time if I wanted, or take the easy way out if I wanted (I usually wanted the easy way). In the same way that my players often aren't all "That feat would be impossible for it to have!", they're also not like "You didn't build this according to the strictest interpretation of the NPC rules!" All they cared about -- all I cared about -- was the effect it had at the table.
But if the enemy is supposed to be kind of equivalent to the PC, I do feel like I want it to be something the characters can envision their own characters doing at some point. If I introduce a thief NPC who can attack, dodge, survive, and hide better than my ostensibly same-level Rogue PC, the natural question arises: "He's good. How can I get to be that good?" And if the answer is "No, you can't, here's some metagame justification," it takes me right out of the moment and also makes me feel like a cheater, taking away something that could have been a cool character option, if only it could be included.
That said, the main difference in 4e between monsters and PC's is that monsters get all of their HP in a single encounter, and PC's spread it out over a few encounters with healing surges. Most monster powers could easily be PC powers (maybe different numbers, probably the same effect) and vice-versa. Solos and Elites are really just "Four PC's" or "Two PC's", and I wouldn't even mind giving some of those abilities if a few PC's worked together to get the effect (in fact, I think that's kind of cool).
It's possible to bridge that gap in 4e, I think. And I don't think I want to go back to "PC's and NPC's share the same rules." I do think I want some overlap, a place where PC's can get things that enemies which are clearly the same general category of creature can get, and that monsters can use some of the abilities that PC's can use. The Chimera and the Wizard don't need to be built according to the same rules, but the Wizard and the Evil Necromancer should be able to recognize some rules elements in common between them, and the Wizard should be able to go off and learn the Evil Necromancer's spells, should they want to.