The whole package.
You'll see that it compares most easily to Civ IV BTS - similar victory conditions, similar diplomacy, wide tech tree, stacks of doom, etc. Except every facet is just done a little bit worse than BTS.
The military and culture tech tree is littered with bland techs which are basically laser 1-5, bigger lasers 1-5, biggest lasers 1-5, laser defence 1-3, bigger laser defence 1-3, replicated again for the other 2 weapon types. When you combine it with the ship-design aspect of the game it just becomes a micromanagement pain: every 5 turns or so you'll get a (very slightly) better version of your lasers/missiles/defences/hull and you should theoretically upgrade your designs. If you're like me and can't be bothered doing it that often, once every 15 or 20 turns is still a pain. Using the pre-set designs is not an option if you're playing at a high difficulty because they're sub-optimal.
Then when you actually get to the combat, it's much shallower than civ IV's. There's no terrain bonuses, no collateral damage and no concept of ground/navy/air. Levelling your ships just gets them slightly improved stats rather than specific promotions like BTS. The tactical depth is almost non-existent.
Cultural victories are done much worse than BTS. A typical cultural victory is done by pumping out cultural space stations and upgrading them. It's not about racing to build the key wonders and acquiring religions while keeping everyone happy so you don't get dogpiled - in GalCivII it's about pumping out dozens if not hundreds of little constructor ships which you have to micromanage to all corners of the universe to build cultural centers.
I won't go on with how overrated the AIs are or how bland the other victory conditions are. You'll see.