1) The variety of content and the constant variation between games. The fact that the game mechanics (combat, UI, navigation, etc) are slick and well designed is also integral to why the game is so good. The interaction between items and events (like mirroring the Kawangi into the Vacuum bomb or a black hole) is also a blast.
2) The imbalanced items. Once you pick up the cloak, the gong or the mantle, you're pretty much invincible and a lot of the tension drains away. The neutronium railgun is also too strong against missile weapons- it's usually best to keep the two you start with over most 3 or 4 star weapons.
3) The large map. It's got the most space to explore gives you the best chance to enjoy conquering the galaxy (that's how I usually play it
)
4) The military frigate.
5) I think the bosses mostly too easy, but in most cases it's due to the fact that they're vulnerable to a particular very common weapon type. Neutronium railguns make fighting tchorak, garthans, and primordius a breeze, for instance. I'd enjoy the solitary bosses more if they were on a level with the Kawangi.
6) I play it risky. It's more enjoyable to do the risky combats than ones that are shoe-ins.
7) Not really.
8) Yes.
9) Babulon.
10) No. I bought the game after enjoying the demo.