Even though I'm fine with the floaty physics and plane switching, the only reason they do ruin the game is because so few user created levels make their levels around them. The end of the first Contra level is a good example of this; if you don't stand on the middle platform, you can't hit the target, even if it seems like it should work on either the bottom or top platforms. I'm not saying it's all user created levels because some I've played are very good, and sometimes this is when they decide to restrict everything to a single plane. I still think the actual story mode levels (and official MGS levels) are really, really good and I never had plane or physics problems with those levels. The creator is simple if you want to go in and mess around with things, but if you want to design a good level, it is really hard.
Something like the Far Cry 2 map creator, or the N+ one, or even Forge are much more simple to mess around with since you don't need to worry about the mechanics on how to make the level work. If you even watch one of the MM done guides on how to create a simple one way switch, it seems so complicated, and that's just to keep a door open.
