I think it's fine to add more monsters and buff them, but then your own toolbox needs to expand, as well.
We have some primitive options for automated defenses. I would love to see more and better ways to weaponize pistons, automate arrow cannons, deploy fire against enemies, etc. I think Minecraft could become a sort of real-time strategy/tower defense game if they put more thought into those things, and it wouldn't lose anything in terms of the Survival mode.
I could do with things like waypoints/patrol routes for wolves, auto-aimed arrow cannons, etc. It's pretty easy to automate mob grinders but I think it's excessively difficult to defend a large structure. With Notch nerfing the Endermen's block-stealing abilities, I guess that doesn't leave any mobs that can destroy your stuff without you being around--Creepers still have to be set off, after all. But I think when they add a new mob, they should about what countermeasures players have against them, beyond the simple "run up and whack 'em with a sword" technique. And if we're going to have mobs that can grief your structures when you aren't around, provide an effective countermeasure. I don't care if you make it kind of costly (so you can't spam them), just make it be something
I would point out that this also makes PvP a lot more interesting!