Tylermon
Perhaps there is a barrel in front of the player. Now instead of hitting it, they might slide all around it. Despite a barell being a solid object you want a player to stop at. Or if an environment has objects or props rotated, now these become sliding obstacle courses.
A solution to your barrel problem - the underlying collision for your barrel example should probably just be a non angled box anyway. If you need it to slide at angles, have two underlying collision objects, one solid that doesn't rotate to stop the player, and one non solid that rotates for pushing. It seems much easier to account for pushed objects rather than coding complex movements. Perhaps to take it a step further, a no slide behavior could be made. That seems unnecessary though.
I'm not sure I understand your "sliding obstacle course" example. If an obstacle is slightly angled, it shouldn't stop you. It's awkward to get stuck on it. Players shouldn't be expected to push, up, then right, then up, then right, etc to move at an angle that's not 45 degrees.
Add too much, and it becomes the platformer movement. Great for that one purpose. Limiting for much else.
The platform movement is pretty awesome. You can mold it to how you want. How is it limiting?