Well, "good" and "bad" people will happily argue about til the end of time, haha. But here's what RT brings to the table:
1. New Diplomacy UI and mechanics. Completely removes all facets of old Diplomacy, if that affects your opinion.
2. Aquatic Cities. Your Mileage May Vary, some people like 'em, some people don't. They function in somewhat different ways to land-based Cities.
3. Improved planetary Biomes that also come with associated Quests based on landmark features called Marvels. Aliens also differ in looks (and possibly stats) based on Biome. Examples: Frigid Biome, Arid Biome. A common complaint is that the Marvel Quests are underwhelming.
4. Artefacts. You acquire these through pillaging Alien Nests or completing Expeditions. Can be combined for boosts to your economy, unique unit upgrades, buildings or even Wonders.
5. Hybrid Affinities. You can combine any two of the existing Affinities to unlock a specialist Affinity with its own units and visual theme. Not as game-affecting as your main Affinity choice, however (only affects how the units look, not anything else).
6. Warfare is now based on War Score which allows for a bit more flexibility when suing for peace. Might affect you as a warmonger!
7. A lot of UI polish. Makes the game feel a bit more alive / interactive, in my opinion. New unit upgrade screen, and other redone screens.
8. Other random stuff like improved graphics for aquatic tiles, new aquatic units, balance changes, Spy Ops and so on. The typical stuff you expect in an expansion.