I'm sure we all have ambitious ideas for levels, though before you start going overboard, take a look at these limits, you might have to change your plans to fit. I'm assuming these limits will be carried over to the real level editor when that comes, I know the object one is said to be staying, maybe one of you Twitter using folks could go ask them about the other ones.
Here's a list of the limitations I've stumbled upon, please let me know if you found any others:
Keys: Only 6 keys can have gravity, all the rest just float in space. This killed one of my first level ideas.
Lights: Only 10 light sources can be placed, if you want tons of light, try using one big light instead of several small ones.
Waypoints: Only 9 waypoints can be placed, if you want an extensive moving platform level, you might need extra platforms. Funnily enough, the actual time amount of the waypoint doesn't seem capped, I got up to 10000+ before I got bored (before you ask, there's a nice way to put a huge start time on a waypoint without killing your mouse wheel, simply select the waypoint you want, start scrolling the wheel, and while you're doing that, click to the desktop or another window, the time will go up rapidly until you click back into the game (obviously this only works if you are in windowed mode))
Objects: Only 400 objects can be placed (this includes set pieces, hazards, and possibly other things, not entirely sure), don't get overzealous with those
Color: This is the opposite of a limit, try setting the RGB values to something over 255, it changes! Thanks to HybridStars for telling me this. You can also set the amount to a value over 1, thanks to RockLeeSmile for telling me this.
That's all I have for now. Like I said, if you stumble upon any other limits not listed, post them here.