The first thing you have to do is go into the video options and disable the Color Correction, otherwise everything has a blue/green tint for some odd reason, and looks way too dark. There might be a certain appeal to that spooky super-dark look but being able to actually see everything realistically is far better, and there are still areas of dim lighting.
I've had a LOT of fun in Warframe's CB for the past couple months now, there have been a few pretty major changes in that time, most of which offered big improvements. For the truly-free player there's plenty of gameplay, and you can upgrade two frames to max level for free as well as plenty of weapons. For non-paying players the only limit is in how many max-level frames and weapons you can craft so you only have fewer options but no disadvantages over paying players.