I have a lot of experience with beta testing, as I've tested everything from small indie titles to big time triple A titles so I know what to expect during and after beta.
To me, warframe looks nice, it uses the great use of lighting and shadows to make the back of your warframe (its a third person action game) look pretty neat. Other than that, everything is looks poor. The enemy models, the map textures and map design are hideous. I have played through the entire campaign that this beta as of January 17th offers and I've seen the same 4-5 maps over and over and over. Its about 50+ missions of the SAME map designs and textures, maybe a little green and blue here and there but its the exact same thing every time.
There are probably less than 9 different enemies, and they all behave incredibily dumb. THIS might have to do with beta, but the lack of enemy types is something to worry about. Enemy types isn't a "beta thing", they all behave and their gameplay is exactly the same. I'm pretty sure the devs aren't going to completely redesign all the enemies, and if they do, its the first time I've seen it. My best guess is, if they recycled the same maps and missions dozens and dozens of times, varying enemies isn't a concern. It would actually break emersion even more being on the SAME exact ship as when you first started the game only now with new enemies. Thats just dumb regardless.
Combat is enjoyable for the first couple of stages when you start unlocking new skills. But when your warframe gets strong you start to realize how bland the game really is. Enemies don't pose a single threat if you're in a team and you have half a brain. Swinging your sword and cutting one in have is cool in the beginning but that gets old and tedius too. If you're solo, the game is SO boring because enemies keep spawning if you don't kill them fast enough and they spawn in random places. You could have cleared out and entire section of clunky fat ugly looking "marines" and all of a sudden they start spawning hordes behind you.
The controls are meh, for action games, I expect them to be a lot more respsonsive and cleaner. Charging up a sword is a chore because some times it doesn't register in the middle of certain movements. So you have to wait until your warframe completely finishes a slash or a roll, stay still for a bit, then hold the mouse button to charge or else it won't respond. If you try the game and you're used to quick actions and movement, you'll see what I mean.
Some animations are nice and some are bad. For instance, climbing onto ledges is incredibily broken. You can climb on to tall boxes and when you do it staggers your character into the climb animation BUT you can't climb railings shorter than a box. You can't climb a RAILING! Think about that for a second....now seriously?
All in all, definitely try the game because you might get some enjoyment for a couple hours and you lose nothing in a free game. But Warframe isn't a boring, repititive instance based game because its in "beta". This is what they are going for, its the same reason why games like Vindictus went no where even though they were super hyped during beta stages. Its the same map, same enemies over and over. Actually, Vindictus had different bosses at the end of their maps so it's actually probably more varied than this recycled-to-hell game. You get what you pay for, unless its Planetside 2 ;)