The strength of 3.X lies in the sheer volume of material which is available - and that's only counting official products. When you include 3PP then it becomes astronomical.
I think that with a game so broad, you really need to decide what you want to exclude, and by only including the elements you want, you can really create a unique feel and experience. And by including everything, you can’t really make something logical and coherent. Excluding even core elements is possible because there are so many other options. Don't want Clerics of Druids? - ban them, there's plenty of alternatives to choose from to fill that niche. Don't want Dwarves? Gnomes? Elves? - no problem.
In terms of what it's good at? Well, anything, really. But it does take a fair bit of work to get to that point, and I think that investment is kind of unavoidable. That said, once you've decided which elements you want to include in your palette, I've found the only real obstacle is physical in the number of books you need to cross-reference.