This guide was made, because I needed more space for my review.
Should the amount of information overwhelm you, which is very probable for a newcomer, I suggest you bookmark out the wiki[warframe.wikia.com], which will be a much needed and absolutely great resource.
However, be warned that you can easily spoil some fun should you dive too deep into this monstrosity of a wiki.
Another reliable source of information and help are the many YouTubers like iFlynn and Brozime.
Should you, however, be interested in a dedicated beginners guide by me, give me a heads up in the comments.

As this tag is often off-putting for a lot of potential new players, because of fear concerning pay to win mechanics, I will tackle it first.
So let me state clearly: this model is in no way p2w.
I hope this will become clear by the end of this review, but you can achieve everything in the game without spending any real money, except for some cosmetic items, although you might get tempted to do so because it is just so good.
Anyway, let me shortly explain how earning Platinum is done in the game and what you should spend your initial seed money for:
You start with 50 plat, which is best used for a) two Warframe slots (20 each) and two weapon slots (12), in which case you need for farm 2 plat or b) one Warframe slot and four weapon slots.
With farming platinum I mean getting Void Relics, opening them in special missions and selling the earned Primed parts to other players, either via trade-chat or external sites like www.warframe.market.
Nonetheless, I won't deny that this game is grindy - because it is and very much so. That is just basically the essence of this sort of entertainment and if you don't like running the same content over and over again, it's probably not for you. However, if you enjoy doing so, the game will reward you with endless amounts of fun.

You might not have expected a story from a free MMO, but here it is and it's amazing.
Playing as one of the Tenno you are able to control the powers of the Warframes which empowers you to fight the warring factions of the Grineer and the Corpus as well as to stop the spreading Infection and help the remaining humans in the galaxy.
On your journey, you will find friends and foes and learn more about your and their origin, history, and reasons.
While progressing through the Star Chart you will also find traces of new Warframes and start quests to recover them - personally, I think this is where the story shines the most and can wake a lot of different emotions.

An open world, this is not - but you can travel to 16 different planets and locations ranging from the overgrown, post-apocalyptic earth inhabited by the Grineer to the Corpus reigned Venus and going to the mysterious Void, with it's beautiful Orokin architecture.
In each of those areas, you have different nodes. You unlock access to them by completing the previous node and after completing an area you fight the guardian of the junction (sadly those are pretty lame fights...) to travel to the next domain.
Often times there will be additional conditions to unlock the junctions, like defeating bosses, collecting resources or completing milestones in missions.

Should you suffer from claustrophobia, rejoice - the Plains of Eidolon are your playground, a more open-world like area with activities like mining and fishing, as well as doing field missions.
Located way too early just behind Cetus on Earth, the starting planet.
At first, you should just ignore it there and go ahead to the junction to Venus and later Mars.
However, after acquiring some decent gear and leveling up you can tackle various levels of enemies here and even find the Eidolons - huge field bosses that appear at night and present a pillar of the relatively weak end game.

I've often used the words Grineer, Corpus, Orokin, Tenno and the like, but who are they? What do they want?

1. The Tenno are the player-controlled faction in the game, being guided by the Lotus who is lovingly called Space Mom by the community and developers alike. However, you will soon enough learn that every mother used to be a daughter once and you can't exactly choose your parents...
   
2. The Grineer are a race of manufactured clones created to be loyal to the Queens and the opposing faction you will encounter first thing in the game, alongside with Captain Vor whom you will have to kill early on, but not for the last time...
   
3. The Corpus are more technically advanced conglomerate, driven by greed. Prominent among them is one by the name of Alad V - a scientist of sorts, supposedly an Orokin descendant, who developed a lot of the grotesque machinery which will stand in your way.
   
4. The Orokin are an ancient civilization which once possessed enormous influence and power, however, only hints of it remain. Hints you will find out are connected to a certain other well-known faction...
   
5. The Infested were once people of the Grineer or Corpus, who came into contact with a disease produced by the Orokin. Long forgotten who they once were, they will try to consume everyone in their way.
   
6. Last but not least, the Cephalons are synthetic AIs assumed to have been created by the Orokin. During your journey, you will meet the most different characters in them, first and foremost your very own ship-cephalon Ordis. Starting out as a pretty annoying piece of software with some serious glitches this fellow will become a strong ally and finally a good friend.

Even though I made some factions out to be your sworn enemies, relationships in intergalactic space are not all black and white. You will find deserters searching for refugee, you will take missions serving one of the factions and eventually find friends among them.

The bread and butter, the core of the game - the essence, so to say.
Warframe is a 4-player-coop third-person-shooter with abilities and hack-and-slash melee combat. All of the above combined with its amazing parkour-like movement system and the physical enhancements of your frame make for a fun, fast-paced horde-shooter.
The choice of playing alone is yours, but to tackle later content I recommend finding teammates, which can be pick up groups or friends you find along the way.
Should you wish to stay even more connected with them, you can create a Clan which also has a whole Dojo to itself.
The advantages of the Dojo are numerous, like having trading hubs with potentially no fee, research labs for weapons and warframes and obstacle courses, to test your parkour skills and impress your mates.
Of course, it is not just a shooter, as I already mentioned and it also doesn't exclusively focus on horde-fighting, but these are probably the most prominent features.
To elaborate this point, take a look at all the different mission types[warframe.wikia.com] - ranging from the fan-favorite Survival, to Capture and Spy.
Additionally, if you are like me and like your weapon variety, check out all the weapons[warframe.wikia.com], which are way too numerous to even scratch the surface by naming a few.
Finally if all the above doesn't offer you enough customizability - check out all the different mods[warframe.wikia.com] you can slot into every part of your equipment.
Nonetheless, the game has a pretty big problem and that is that even if you have all your shiny, fully modded gear, you will get to a point where there is just no challenge worth taking anymore - just coming to that point first, will take quite some time. The whole loot system revolves around rotations. There are loot tables[n8k6e2y6.ssl.hwcdn.net] A, B, and C, each with different items associated with them. The endless mission types like defense, excavation, and survival always rotate like this: AABC, meaning 5min into survival you get a drop from table A, 10 min A again, 15 B and 20 C. All the while enemies keep scaling, but the loot resets - so it's more efficient to just start over again.

This game looks stunning, even more so if you consider that there is no entry fee to it.
Moreover, it has a great diversity of visual facets - I already mentioned the earth-taken-back-by-nature and the golden Orokin tower. However, it doesn't stop there - you have underground laboratories, the desert-like surface of Mars, the annoyingly endlessly long, icy hallways of Europa and so much more.
Nonetheless, it comes with a catch - you can only play one of these tilesets so long before it will get repetitive and boring.
On another note, I can't forget to mention the fluidity and amazing quantity of animations, from jumping to climbing, over rolls and flips in every direction. It just feels natural performing all those different maneuvers.
Atop all of this, is, in my opinion, the customizability of your Warframe, being able to choose from many different colors for 5 different color-zones for your Frames and Weapons.
As the half-joking saying goes "fashion frame is endgame".

The developers took the sound so far, as to make it a gameplay element for one of the Frames, for which you can compose your own music which scales with the noise of the surroundings, increasing the damage you deal.
Nearly every weapon sounds different and some do so outstandingly, packing such a punch that you can't help but grin as you obliterate your foes, as they cry in despair and try to snarl commands at their peers.

Thanks for bearing with me misusing the guide section again.
Should you find my rambling lacking, tell me so in the comments and I'll see if I can do something about it.