You can remove the top lid by unthreading the webbing through the ladder locks--lighter weight and simpler than buckles.
The sloping bottom was on purpose--to minimize the amount of pack sticking out behind you when crossing rough terrain where rocks might bump your pack and throw you off balance. But there are lash points where you can attach a pad which will make the pack stand up, so really it's a win-win.
The stiff foam will last longer than soft foam that will break down after just a few years of use, so what's better... cheap, soft foam that won't support a load or harder foam that will last for years?

I agree with mtn that the top lid being removable but i found that I would need to cut the sewn ends off of the straps to get them through the ladder locks.