Three things that must be allowed in the new version:
Template application: If I want to make a vampiric spell-stiched unicorn, and I've got the story to back it up, I better be able to do it!
Leveling: Every intelligent beastie (humanoids and a few other types) in the MM needs to have the ability to level up or advance in some way.
Advancement: Every non-intelligent beastie needs to have some way to advance and become more powerful at the DM's descretion. This could be through the HD advancement in the current edition or something similar.
If this stuff on "ranged" and "tank" monsters bears out, I just want some customization options. If I want a crossbow wielding ogre, why can't I?
And, honestly, talk of trimming back abilities has me worried too. The more "powers" a creature has the more it works in my stories. I know what a mook does. I can make mooks from here to eternity. I need the uber-bad guys with a heap of power, not someone designed explicitly to last 5 rounds. Where's the role-playing in that?
Now, this just me reacting to chatter, so all these worries could be totally unjustified.