A bit more time, a bit more information!  I will go over some high level points first.

• Aside from a few trading hubs, and the clan hall (dojo) everything is instanced.  Think D3, or Neverwinter dungeons.
• Chat in combat is not really viable.  Voice is the way to go.  Expected in any FPS really.  General chat is a mess (as in every game).
• From what I can tell, nearly everything is free in this game, minus customization (icons, colors, skins).  There is a LOT of free customization, but the bigger ones have a cost.
• Acquiring new items is time gated.  Hours up to 3 days.  Can spend money to speed it up.
• There are two ways to progress.  Item levels, and character levels (mastery).
• There are many items.  Warframes (classes), ranged, melee, small fire weapons.  It’s actually hard to keep track of it all.  Each can level up to 30.
• Each item can be modified with Mods.  The level of the item determines how many and how strong the mods are.  Mods impact pretty much everything you can think of – speed, damage, crit, health, elemental attacks…it is very complex.  Adding and removing mods is free.
• Mastery has a cap of 29.  It is raised by getting more level 30 items (and some small mini-quests along the way).  It opens up more of the game, and capacity.  It also gates progress as some items have minimum mastery levels.
• Mods are life.  You can upgrade them.  You can mix them.  You can make a speedy crit master, or a super tank.  Some enemies are resistant to X, others are not.  You need to make conscious decisions before heading to missions.
• There is no progress under regular MMO terms.  Assuming you have access to all the level 30 Warframes, each Warframe has a specific use for a specific mission type.  Each weapon is the same.
• I have played FPS before.  This is not a ranged FPS.  The rooms are generally tight, enemies run up and there is a lot of opportunity for melee.  The parkour movement makes defense more about not getting hit/always be moving, rather than soaking damage.
• The actual controls are ok.  It takes a few hours to get used to the rather insane speed and to get your eyes used to what is around you.
• The art-style works for me.  Mileage will vary.
• There is actual lore, though mostly gated through quests.  You can scan everything and their mom to build an information log though (and there’s a faction for this too).
• Factions (syndicates) are present, with reputation gains.  Aligning with one may impact another.  Poor rep causes hit squads to come after you.  It’s like Vanilla WoW factions more than factions as we see them today.  There’s a choice to be made. Yay!
• There are bosses to farm.  Bosses are quite different.
• Each planet has 10-12 missions to complete.  Leaving a planet requires achievement based goals.  e.g. kill 150 enemies in 1 mission, kill a boss, scan 3 statues.  You can’t just skip to the last planet.  There are good and bad aspects to this.  It does prohibit catch up work, but item and Mastery levels are the true gates.
• The Archwing is not good.  It’s a 3d space sim shooter.  It is bad for 2 reasons.  First, it’s in 3rd person, which means you can’t see half the screen.  3rd person is designed for forward, one axis movement.  Second, the radar/map is not built for 3d combat, so it doesn’t show anything.  You end up getting attacked from all sides, with no notice, and no ability to see them.  Thankfully, this thing is not critical for game progression.
• There are a dozen mission types, then variants of those mission types.  Assassination, decryption, tower control, rescue, wave defense… all but tower control can be soloed.  Variants adds different enemy types, objective modifications and higher rewards.
• Default LFG for every mission, for up to 4 people.  You get a bonus to everything in a group.  It is a great way to experience the game.  Solo if required for some specific objectives.
• Tutorials are not good and do not do justice to what’s in the game.  You learn by playing and asking questions.
• The combat, art, and mechanics are polished.  Way more than I had ever expected.  This thing runs super smooth and responsive.

High level…. yeah right.

The important thing to remember is that there is always something to progress towards, always something to do.  It’s as if you mixed an FPS with an action RPG.  Think Hellgate for those that played it (I did).  The lack of focus or general direction can make it challenging for some people, but if you like settings your own path, this scratches a bit of that itch.

I hit Mastery 3.  I’ve yet to max any item or warframe.  I have a pet that shoots ice beams.  I shoot fire bullets.  I spin through the air like a dancer, and land with a giant axe.

I am having a lot of fun doing it.