This game has been there for me for a long time. I have more hours than just casual play but not as much as a diehard. Here is what I think of the game for the time I have been playing.

As the Devs have stated the game is meant for the early game and new player experience but as of recent they have been adding late game things to do that I am very thankful for so lets start at the beginning.
Every planet boss drops a frame that you can farm all the parts for which is highly encouraged to get an idea of what frames you like as they don't take many resources. Since they updated the star chart they added little relays you have to do a few things for to get to the next planet. As this might not seem significant early game each one you complete gives you mastery and free resources and blueprints. From there you are free to explore you're next planet and so on, but for all the newer players I would highly recommend playing the open world(ish) areas The Plains of Eidolon and Fortuna as they can offer as different experience, but you won't get to fully experience them until once you have completely beaten all the available story missions. On the other hand if you start on them early you can create powerful weapons that could actually carry you through all of your missions and bosses but might also ruin your experience of how difficult they could have been.
As for the other things there are the Void, Orokin Derelict, Fissures, and Sanctuary Onslaught. These are all different things that have their own uses even to a player to the point I am at. The Void is the ONLY place where you can find Argon Crystals which is a resource that decays but is used in a variety of weapons and frames, thankfully not all of them though. The Orokin Derelict may not seem special for the first few runs until you find a door with four key holes that you can't open, these are vaults. They can only be opened with dragon keys and drop rare exclusive vault mods when they are opened, but beware that the keys have different things inhibiting you. Fissures I won't cover much as to say they are random missions of varying level where you can take in a relic to open from killing specific types enemies and collect prime parts and blueprints from. Sanctuary Onslaught is a single room of a tileset that you are placed in with your team and must kill things as fast as possible to not let the efficiency drop to 0%.

Overall I still think the game is fun, I know that some people would argue there is a pay wall but honestly if you can grind out prime sets and get vault mods along with riven mods (I didn't discuss these or sorties) you can easily make platinum just by trading with the community.