Variety
Even with only 46 units so far, I´d probably never run out of ideas for army building. The units are both unique and well balanced against each other, so each game I have a new idea to try.

Short and to the Point
The average game lasts 7 turns, so can be finished in a week with 1 turn/day. No drag. I am not really into games that take weeks or even months to finish.
Pure Skill
The spread of random outcomes in fire and melee is too small to decide a battle. You have also little to fear from the scissors/stone/panzer dilemma, if you build a fairly balanced force. Skill decides every game, all the time.
Every Little Thing Matters
Battlesector rewards careful play. You can´t simply lose a couple figures here or there by negligence and expect it not to influence the outcome. Every unit and every action matters! Interesting decisions constantly.
Units and Terrain Look Great, Good Voice Acting
What I usually care least about, but together with the fantastic gameplay it´s the icing on the cake!