The only thing I find disconcerting about this is that it reduces a pit fiend to a one-encounter thug. What happened to the evil mastermind, one of hell's top men (top. men.) who plots and plans and maneuvers and outthinks the PCs.
Let's look at an example from my home game: I had a blast running a siege adventure a few years ago in Hawai'i where the general of the evil forces was a pit fiend. The array of interesting spell-like abilities it had made the entire siege interesting. The pit fiend had a lot of re-occuring villain staying power (since he waaaay out-leveled the PCs at the beginning of the campaign-arc, but eventually they caught up to him and gained the items they needed to put him in his place). That whole series of adventures was absolutely delicious and the party were EXUBERANT when they finally killed General Zar'kith. I could not run that campaign with this pit fiend. Instead he feels like a one encounter chump. And that's fine for most monsters, but not for a pit fiend in my humble opinion.
Finally, I know you're gonna say I can add stuff or thingamajigy with him...but then what's the point? Do I have to do that everytime I want more than a one encounter meat puppet to throw at my PCs? If so, well then that sort of makes the KISS point moot. In 3.5 I could just pull a pit fiend out of the MM and there's my kick ass mastermind villain. AND! Really folks, the spell-like abilities are NOT tough. They are spells! We all know the spells! The PCs have em too. It's not like the pit fiend in the MM has a gajillion weird powers we never heard of:
blasphemy (DC 25), create undead, fireball (DC 21), greater dispel magic, greater teleport (self plus 50 pounds of objects only), invisibility, magic circle against good, mass hold monster (DC 27), persistent image (DC 23), power word stun, unholy aura (DC 26); 1/day—meteor swarm (DC 27).
What's the big deal? A lot of baddies have the same abilities, and they are pretty easy to prep.
There's a lot I'm liking about the new edition from what I've seen, and I think this format works great for SOME monsters, but I'm personally not digging on this version of the pit fiend as much as 3.5's.