For me, the following make a good space naval combat game -
1. Good background story, or at least one that is relatable and open to fine details. Full Thrust wins in this case, as while there is broad mapping of major powers, minor forces fit into the joints and allow players to fill in with their own ideas.
2. Variety of weapons. Not every species will reach the same conclusions about what technology is best, so there are going to be differences in technology. SFB and Full Thrust have this and it works well, because nobody has Uberweapons but forces aren't simply same-old same-old. To some extent, Colonial Battlefleet also does that as well.
3. Ease of play. I don't want to simulate exacting mathematical calculations of movement, though I'd like some of the basic physics to be included, as FT does rather nicely in both Cinematic and Vector movement versions; Starfire doing the same as Cinematic and justifying it with its technology. Damage Control and determining results of fire is also how I decide on a game system, and FT is again a big winner, though CBF and even Firestorm Armada come close there.
