The sector system works spectacularly well, so big kudos for that one. It feels like islands in civ games - the thrill of exploring a new island vs just bland homogenous land mass. Easily the best part about galciv4.
I find the economy system really hard to understand and optimise for. Building planet upgrades takes forever, and feels like it adds such a marginal gain right now. I think this could use some more work. In theory I like it, but in practice it feels like it needs something more.
It would be great if citizens had their own special powers (a bit like leaders)
I really like the colony vs major planet system, its great, but as someone else said, it has one huge downside - I spent most of my game spamming colony ships. Not fun at all. Maybe one way to fix this is to let you colonise an entire system at once with a special colony ship available in midgame.
The new races are great
The new tech selection system is a bit annoying. I like it in practice, but I think it needs to provide some balanced options. I didnt get presented with a single military tech option on first roll for a 10-20 techs in a row... not great when the Yor are threatening me and telling me how puny my navy is...
I like the new random events, but you need to add a lot more - the same ones keep coming up over and over. I think you probably need 200-300 minimum. Add more quests too!