Bungie i pretty good at making interesting aliens. Look at Halo. The Covenant , Forerunners, and flood were all done well. I doubt Destiny will be any different.
I get the feeling that we'll find out more about the aliens in missions beyond the beta (For those who don't know, the Beta imposes a level ceiling at 8 and only allows access to the Old Russia levels) I was actually just starting to get interested in the story when I hit the beta limits. Very frustrating. I find the Fallen intriguing, (That the lowest caste "Dregs" of their society gets their lower set of arms lopped off in a shaming ritual and their apparent subservience to those "Servitor" AI things interests me, but I want to know more, much more) I'd also like to know more about how Ghosts choose Guardians from the dead and resurrect them (And the story behind the Awoken), but that's the sort of thing I would expect from a accompanying novel honestly.
I downloaded the Beta over the weekend and had a blast with it. Don't go in expecting an RPG, but the character creator was quite satisfying. I maxed out a male Exo warlock within the Beta limits and started up a female Awoken hunter. I even amassed enough Glimmer to purchase a new ship for the warlock (Most expensive one available, purely cosmetic, but prettier than the rust bucket starting ship). Very few games(Especially shooters) actually manage to get me interested in the character advancement, but Destiny has managed it. It's not just better stats, advancement opens up interesting new abilities to make use of and experiment with. I even started experimenting with multiplayer, which I hardly ever do, and had fun. Sure, I died a lot, but I found myself getting better at it and having fun.
Destiny has my seal of approval so far.