@quemador: The game is "grindy", but in the sense that any loot game is grindy. It's not much different than your average MMO or Diablo. (Or Destiny.) It may not be immediately obvious, but Warframe is pretty much all about the loot grind.
The best thing about the game though is that you don't have to pay a single cent to access any of the features in the game. The vast majority of the stuff that requires their payed currency is only cosmetic. But even that stuff can be obtained with out actually having to pay any real money, since you can trade platinum with other players. You can make a lot of free platinum just trading stuff you find in the game with other players. I don't know of any other F2P game that lets you trade their payed currency with other players, and I have to respect them for that. They have one of the least scummy F2P business models I have encountered.
Just off the top of my head, Spiral Knights and Guild Wars 2 also let you trade for the paid currency :)