Hey Scribble, just a quick note... if in the 2 DMG's and/or 2 going on 3 MM's 4e had given us a breakdown and rules for comparing and balancing powers... I would be right there with you... but it hasn't, so in essence even with powers replacing feats in 4e (Which is not something I necessarily agree with, especially as we get more feats that do more than just grant a minor bonus) IMO, we are still lacking in the customization aspect of 4e's monsters and NPC's.
But there are guidelines.
When I create or customize a 4E monster, I know around what attack bonus I should have and how much damage an attack should do, based on whether it recharges, what type of monster role I am in, how many targets it hits, what defense it attacks...
Now, there aren't as useful guidelines for adding the extra features - what conditions it inflicts, what other elements the attack might have.
But I have found, with those guidelines, I can make pretty informed decisions. I know that whatever I choose, I'm not going to have an attack that just blows PCs apart, or bounces off them harmlessly. I know, based on the level alone, the monster will be an appropriate challenge in terms of defenses and hitpoints.
Which - at least for me - are far more useful reassurances than the entirely by-the-book monster customization formulas of 3.5. The CR system really did fail me, time and again. Leveling monsters, advancing hit dice, adding abilities - I often found that I could trust the end result. Yet the book said I could, and so I went with it anyway. It didn't fail me every time, of course - but I regularly found myself comparing newly adjusted monsters to ones from the book anyway, and trying to scale them appropriately from there. Despite all the formulas, monster adjustment was an art more than a science.
And thus I was very glad when 4E outright acknowledged that. You get guidelines and pointers, and the advice to check your work anyway. You do have some templates for adding classes to existing monsters, or NPC rules for building humanoid NPCs from scratch. But if you don't want to go by the book, you can also just take a monster and swap some of its powers for appropriate level ranger powers.
And that is the real strength - the time is takes to make those adjustments is far less than I would spend statting out high-level monsters or NPCs. And more basic changes - like adjusting a monster's level up or down - I can often do on the spot. Combined with the expanded base options for many monsters... I find I prefer 4Es approach, and that it provides all the customizability I need.
I have yet to find myself in a situation where I wanted a certain monster or type of monster, and couldn't put together one, and one that was distinct in what it could do and how it played. And if anyone is really concerned that the DM might get it wrong without strict rules to ensure balance... the fact that it takes less work leaves you plenty of time to compare it to existing monsters and confirm whether a new ability is appropriate at that level or not.
Will the system be perfect for everyone? Of course not. But I've found it is easier for starting DMs by providing them with a more diverse selection of distinctive monsters, and it is easier for advanced DMs by putting the power into their own hands and giving guidelines, rather than absolute formulas, to create and adjust monsters as they desire.