We took a poll of 866 potential Galactic Civilizations II buyers on which feature they most wanted in Galactic Civilizations II.

The results were:

Ship Design: 38%
Fleet Battles: 21%
Better Colony Management: 15%
Multplayer: 12%
3D Engine: 8%
Space Terrain: 7%

So we looked at these things when deciding what features to put into Galactic Civilizations II and looked at the cost of developing them.  On a scale from 1 to 10 with 10 being most expensive here is how much these features cost to put in:

Ship Design: 5
Fleet Battles: 4
Better Colony Management: 5
Multiplayer: 10
3D Engine: 3
Space Terrain: 2

The big new features of Galactic Civilizations II are:

The ship design, which we think goes further than any similar game (or any strategy game for that matter) ever.

A completely new way of handling colonies:

Fleets and Fleet battles (still in development)

And we needed a 3D engine anyway to implement much of the other things we wanted to have in there (resolution independence, fluid zoom-in out, much fancier ship design options, etc.