Pit Fiend
This, my friends, is crunchy bits of goodness. They know full well that this little bad boy will sell them books.
To all the posters that don't like the changes, but haven't ever used a pit fiend: Please don't complain. I respect your right to an opinion, but your opinions don't hold as much weight in my eyes as those of posters who have run such games. I'm not singling out anyone's post, merely making a general comment about opinions on high-level play.
The high level monsters in the Monster Manual were very poorly designed in my opinion, with the exception being Dragons. It was abundantly obvious they weren't playtested well. As a result, I'm confident we'll see these types of changes to many of the higher CR creatures, but very few changes to anything CR 10 and below (other than the mechanics previously addressed).
One thing still missing, that I'm hoping will be available somewhere on the "page" dedicated to each monster are the stat blocks that include information on the "buffed" equivalent for the creature, such as with the Unholy Aura and other spell-like abilities that are necessary to compete with heavily buffed high level PC's.
Additions of different types of AC's and common types of attacks (standard, full, grapple) to the stat block are very nice additions. Thank you!
Why were these changes needed? Where did the numbers come from? Playtesting. I used pit fiends and other fiends, in my games and the numbers on this guy (hit points, AC, and SR) are very close to what they've developed. Close enough that I can say confidently they were not arbitrarily "pulled from the air". On the contrary, these are the appropriate levels required to truly challenge a 15th or 16th level party. Anything less is a joke, and will get summarily steam-rolled by the amazing offensive firepower of a well equipped high level party.
As it regards the lowering of some of the resistances. Bravo! The wizards in my campaign were often highly frustrated by the combination of "high saves, high SR, and high energy resistance". As a result they turned to save/die or save/hold spells to forego the energy resistance piece. By lowering these to more reasonable numbers, a well placed Cone of Cold or Chain Lightning may be a worthy competitor to Disintegrate or Hold Monster.
On the thought of all the existing "3rd Party Material" being uncompatible. Hold on a second! Two ways to look at this. The character classes haven't changed significantly, so you can still use all the encounter, etc, as they've drawn them up. No real significant changes (maybe some with spells). Who cares if the Skeleton has DR 5/Blunt or DR 5/+1? Run it! It will work fine. They're going to get blasted by your cleric anyway! I see this as completely the opposite. Here's a great opportunity for everyone who's produced something to date to send out an "update" which could be free, or...
could be a nice little unexpected revenue boost for not a ton of a additional work. Just a different perspective on what many have touched on.
IE, see Creature Collection "revised"...)
Monstrous multi-classing. I love it. It works much better than ECL's. Now, let's see if they figure out that the next challenge is how do you effectively spread out all of those abilities to ensure that someone who takes one or two levels of outsiders gets "good" abilities, but not significantly better abilities than someone taking two levels of fighter or a prestige class!
Can't wait for more 3.5 goodness. I'm very pleased with the new Pit Fiend, top to bottom.