Well.. it was the feature list combined with execution. It's a 6df semi-newtonian space sim, with linear and rotational momentum, allowing for some really fun dogfighting. It also stood out for stuff like moving around while nanning a bomber, which could well lead to you flying backwards and sideways relative to him, while watching your HUD to see which of the other repair ships the enemy is trying to shoot down, so you can repair him.
For most of the time, when you are not dogfighting, the F3 command view and orders capability let you control you ship while paying heavy attention to the game, and the autopilot along with in-game cursor use lets you figure out what's going on all over the map while you're off exploring, defending miners, escorting constructors, en-route to your destination, forming up with teammates..
The RTS elements are very functional and well excuted. Allegiance is much more a multiplayer strategy game with dogfighting than a pure dogfighting arena shooter. At the same time, it doesn't take away from the individuals flying the combat ships.. it's up to them to execute it, and many the game there is that was swung on a particular player managing to kill heavily defended miners, or capture or destroy a base at an unexpected time in an unexpected way.
It's basically an action game where brains are the most important element, with serious teamwork, and in that, unique.