Well, I'm fairly new to BPs, but not new to small animal enclosures. Here are some people-friendly features I would love to see combined:

Lightweight for ease of cleaning.
Easy to increase/decrease ventilation to control humidity.
The ability to attach toys/decorations in a variety of ways (for instance, in a tub you can drill holes to attach things).
Smooth surfaces--I definitely second that one.
Security and ease of opening combined--second that one too.
Flat floor (tubs often have a concave/convex floor that eats up extra substrate in trying to make it level).
Straight sides (as opposed, again, to concave/convex sides of tubs).
Feet to lift the enclosure high enough to attach heat tape and still have a slight gap.
Feet that are at the far corners and spaced far enough to allow room for 11" flexwatt tape.

That's all I can think of right now. I'll post more if I think of more.

Heather