Warframe is a online co-op game which features Space Ninjas. That alone should be enough reason for you to go and play it right now but I suppose some of you need more than that. It's a third person shooter with great looking animations, design, acrobatic movements and a whole lot of cool stuff.
Some people say that Warframe resembles Mass Effect 3 Multiplayer but I disagree. Warframe offers much more mission and level variety than ME3 multiplayer. Aside from that, Warframe also doesn't use a cover-based mechanic, which makes Warframe much more fast-paced and doesn't promote static defensive gameplay(unless it's a defense mission of course).Warframe also offers a lot more variety in terms of weapon choices. Also, you don't get to do crazy acrobatic parkour stuff like wall running in ME3.
The overall feel outside of combat actually reminded me of Monster Hunter, where player go on missions and try to get materials to craft various equipments, in this case weapons and warframes. The game runs the risk of boring its players with repetitive gameplay but just like Monster Hunter, Warframe gets a pass because it offers fantastic combat and gameplay. Because of it, even when you're doing the same thing over and over again it still feels great and it still looks great.
Of course, Warframe has its share of problems. The UI for the Mod management is pretty terrible. In the beginning when you only have small amount of mods it's pretty manageable but when you have thousands of mods and multiple copies of every mod, finding one particular mod to attach to your weapon or warframe becomes a treasure hunt. Of course this is only a minor problem and one that (I hope) will be addressed soon.
The Mod system itself is interesting and pretty easy to understand, even for new players. The number of available mods gives player the ability to mix and match mods to create various builds. Whether you stack up on damage mods, make your weapon reload faster, faster firing rate or just having loads of ammo it's up to you to chose which mods fits your playstyle.
With the amount of weapons available in the game, weapon balance is surprisingly good. There are of course some underpowered(or overpowered) weapons but Digital Extremes seems to be doing a very good job of keeping things balanced and these unbalanced weapons will be fixed soon enough.
To keep things interesting, Warframe has an alert system which creates a random event every now and then. It's usually around every 30-60 minutes. Alerts are just missions that are only available for a set amount of time which mostly rewards credits. Some alerts have better rewards like blueprints for alternative helmets, weapons, rare warframes or even useful items like the Orokin Catalyst(used to supercharge weapons), Orokin Reactor(used to supercharge warframes) and Formas(used to add polarity slot to a lvl 30 weapon or warframe).
Alerts are not enough to keep you interested? DE will also be hosting some weekend events every once in a while. These Weekend Events are connected to the lore and story of Warframe and success or failure will affect the game in a real way. These Weekend Events awards casual and hardcore players alike by giving rewards to everyone who participates. Of course those who contributes more will get more reward but those who can only play a bit on the weekend will also get something for their effort. In the end, this IS a co-op game so everyone who helps out should get something.
People have criticized Warframe for the lack of PvP but I actually like the way it is right now. Taking their focus away from PvP enables DE to make a better PvE experience and create more content for what Warframe does best, co-op PvE. Putting PvP(or rather, "proper PvP" as there is already a kind of PvP in the game) will take away precious resource and time that could be best spent on improving Warframe even more.
Update 8 added a Clan Dojo to the game where clan members can work together and pool up resource to build their dojo. Dojo isn't only an e-peen thing but also give access to clan weaponries that can be researched in labs. These weapons are usually weapons that are used by elite enemies and now the players can have access to them too.
The microtransaction side of Warframe is somewhat fair I suppose. The only thing you actually "need" from the market would be Warframe and Weapon Slot and that's if you really want to collect every warframe and weapon in the game. Other than that, every weapon, warframe and item are obtainable by buying blueprints, loot drops, mission reward or alert rewards.
Some people still consider Warframe's microtransaction to be Pay to Win but I would disagree. Unless your self-worth comes from having a virtual gun 1-3 days faster than someone who paid(and that someone works WITH you in missions, not fight AGAINST you) I don't see why having something a wee bit faster is pay to Win in a PvE co-op game.
Conclusion? Warframe is a totally great game to play alone or with friends. Great visuals, good music and sound effects, amazing gameplay and frequent updates really makes this one of the best Free to Play game there is. DE also listens to the community and always make changes to improve the game. If you played Warframe before and got bored, I suggest you jump in again and try it out because it's a totally different game even from last month. If you haven't played Warframe, go download and play now! You won't regret it.