Excellent, balanced gameplay
AI that doesn't cheat (unless you count being incredibly fast at micromanaging)
choose your own music for menus, gameplay, action gameplay
scenarios for single player gameplay
lots of maps, for 2-16 players
active lobby
they host game servers for for for $0 (but seriously, please donate)
in-development features that can be enabled with a click and tested
ridiculous features, so you can do different game modes
still under active development and expansion
awesome community
physics-based gameplay - that means, shots are actively rendered. Beam weapons do damage while on. If something drives into it, it takes damage. If you hit your own guys, they die. If you put shields around one section of your base and not another, the plasma cannon rounds might just bounce off and hit your stuff anyways, if it comes from the right angle to do so.
radar has line-of-sight - i.e., hide behind a cliff face and advance, and place your own radars well.
rock-paper-scissors-lizard-spock. That is - air, sea, and ground units, each with unique advantages - but also, amphibious units, hovercraft, long range vs short range, fast vs slow - deep strategic complexity.

Negatives: