So far, I have enjoyed the game, but the coop part of the game feels a little lackluster, it seems that you dont really need your teammates to help you kill the mobs. The only time you need them is when you get downed or you need to open a door. But I guess the coop part of the game gets more important on higher levels?
Late levels start becoming very difficult solo. Defense missions past 30 are pretty much right out, because you simply can't keep the objective's HP up under all the incoming fire. You might get through a wave or two, but it's a losing battle.
High level bosses are an issue solo because they'll eat up stupid amounts of ammunition, and almost all of them will chew through your shields in melee, one way or another. I don't think I can even solo the Hyena, with my maxed out Exacalibur and best weapons. His shield regen is simply too high for one person to deal with, and he unloads on you as soon as he gets a clear line of fire.
Other higher level missions can be touch and go. I've soloed most of the game up to the outer rim systems (Pluto, Sedna, Eris). That's where missions can start going pear-shaped pretty quickly. The game starts throwing ridiculous numbers of guys at you, and all the BS interrupter enemy types start drilling you, while the trash hoses you down with automatic weapons. You have to be very tactical playing solo at Lvl 30+.
So yeah. The only mission I've had any trouble with, with a group of friends, is Kiste on Ceres. It's a mobile defense mission that seems hard coded to throw epic shit tons of Grineer at you. The last objective is very, very hard to keep alive, and people with sub 30 Warframes will get taxed heavily for playing stupid. On the plus side, you can easily pull 3000+ experience a run out of that map.
Other than that, Defense missions can just go on, and on, and on....based on the map, it can get harder sooner or later, but eventually that shit gets out of hand, even for a group of 4 people on voice comms with maxed out Warframes.
Groups are basically required if you want to see the game at its most intense (I think you tend to get more loot overall as well.) Solo does a pretty damn good job of it, but things are at their craziest on a Defense mission at Wave 20 or more.
Finished my Ash Warframe. The bug where Shuriken does nothing 50% of the time when I'm not hosting the game is really annoying. I can't tell yet if Blade Storm is really a replacement for Slash Dash. It takes so freaking long to complete. Solo, I can see it being very handy. In a group, people seem to be done killing stuff and moving on by the time I come out of it.
It also has TWO polarity slots for two, incredibly rare, incredibly specific mods. Grrrrrr. If I"d had this frame prior to Update 8 and Forma Upgrades, I'd probably have ditched it already.