I like to have quick access to two camera with attached lenses. One camera wears a 70-200 while the other something smaller like a 17-70. I also like to carry a speedlite and one other lens (like a 10-22).
I like sling bags, I just need one that can have 2 cameras in it with mounted lenses ready to go for quick draw shooting.
I would also like it to hold a computer (if not, that's ok)