Don't listen to old reviews. Warframe has improved a lot since its release, and it's proving to be a fantastic game.
Well, after that tiny rant, let me tell you that Warframe is an incredibly fun game. I've been with it since the closed beta, and I've witnessed how it started as a rather dull and clunky experience to a very solid TPS. In Warframe, you play as a Tenno, a mysterious race of warriors who are the only ones capable of using the Warframes, powerful combat armors. The whole aesthetic of the game is centered around some sort of space ninjas theme, which is not only unique, but also quite tastefully achieved.
Gameplay-wise, the game, at its beginnings, suffered from lack of mission variety. That, as of now, has been solved. The game now offers Raids (missions where the Tenno have to destroy the level's power core), Sabotage (huge maps that involve destroying sensible machinery), Spy (where you have to extract information and carry it in its datamass), Rescue (find and extract a prisoner), Defense (defending from waves of enemies - more on this later), Mobile Defense (glorious battles where the defense point varies thrice per level), Exterminate (erradicate all enemies), Capture (find and subdue a VIP). These, of course, are variations on the "Get here and do something then leave" theme, but they feel different enough like to have a personal favorite, for instance.
On the other hand, there's the issue of level variety. When the game first launched, there were only two tile sets. The game had around 10 modules for each tile set to build its level from (the levels in Warframe are random, รก-la Diablo), which, of course, was repetitive as hell. However, now there are something in the range of 25-30 per tile set, with 5 tile sets and more coming. Sure, there are still some whiffs of repetition, but nothing really worth concerning about, specially since Digital Extremes has committed to continue adding content.
The gunplay has evolved a lot as well, with the introduction of new weapon types, like shotguns, semi-automatic rifles, sniper rifles and even bows. There's also a wide array of melee weapons, like bo-staves, knuckle dusters, axes and dual-wield swords. Everything is craftable, and drop rates and drop amounts have been increased a lot, making grinding relatively painless and almost unnoticed in the normal course of playing. This, of course, justifies the Mobile Defense missions, where the amount of enemies thrown at you simply makes it impossible to leave the mission with at least a couple of levels and 10k credits in your pocket.
One of the new features that I enjoy the most is the Dojo, where a clan of players can build and customize their very own house. All of the clan members can contribute resources to the rooms of the Dojo, and there is research that can be done in the Dojo labs for unique weapons only obtainable through this mechanic.
And speaking of mechanics, the whole mod system has been revamped. Where before was a sprawling and rather confusing tech tree, now there is a clean and elegant "card" system. Each piece of gear in the game has an amount of mod slots, which may or may not have a polarity, and an amount of mod points. Each mod has an effect on the base stats of each piece of gear, be it increasing Warframe armor, or adding elemental damage to a weapon. The polarities are special slots where a mod with the same polarity will have its mod points requirement reduced. Rarer mods require more points. However, what really makes this mod system interesting is the fusion mechanics. Is possible to take two duplicate mods and fuse them into a better version of the mod. There are also fusion cores, which are like wildcards used to fuse with any other mod, regardless of polarity. This way, no mod card is useless, since it is always possible to use it to feed one you're interested in upgrading.
All in all, I find Warframe to be a very fun game, with great depth in terms of crafting, building a Dojo and modding, which give the carrot at the end of the stick of this particular P2P a very appealing look worth the ride.