I've been playing for a while now - got a guy about level 25. I am impressed that they are replying to feedback often on their forums and updates to the game are almost on a daily basis.
It gives me hope that they can turn the game into something great with the potential, because right now...
I really dislike the energy, power, and skill tree systems. The Warframes all seem very similar outside of a handful of abilities that often are simply unnecessary/useless. Healing skills are targeted on enemies that die with a single attack, an ability lets you run fast when there is usually no reason to, etc. I think the game is at a stage where they can just revamp the whole thing and get away with it. Make the power system seamless with the acrobatics and gunplay at a faster pace. Make the skill trees really emphasize the uniqueness of the Frames. Get rid of the limited energy supply and work out some sort of cooldown system in its place (certain abilities would have to change obviously - and for the better).
The nickle and diming is also irritating, but I think they are going a long ways to improve this as the "reactor" drops are demonstrating.
These are the guys that helped make Unreal Tournament, and yet the guns are so boring! Why do they all shoot bullets!? You got the standard boring frameworks of guns (sniper, burst rifle, shotgun, etc.). Where are the flak cannons? Shock Beams? Why do the bad guys shoot lasers and throw shockwaves but we don't!?
Again - I post the same thing on the forums and I really think it reaches them, so I still have great hopes for the game. I even decided to do the Founders thing.