First I recommend watching the dev streams...
https://www.youtube.com/user/stardockgames
They actually are playing the game to which we have access. July 11th's stream is our current alpha build.
To answer the specific questions you have
Explore... yes we can create large maps with some limited shapes.
Colonize planets... yes we can colonize planets and build up factories / research centers / farming facilities / etc...
Build ships... yes you can use the ships you design and build in the shipyard.
Planets... Part of colonizing yes you can build improvements on planets.
Combat... combat is not really implemented beyond you collide and a pop-up says, you want to attack... yes and combat is over. A basic auto-resolve if you will. But they are working on this aspect.
There are anomalies and asteroid, nebulae and dust clouds (which have an effect on your ships). The computer AI is a work in progress, but you can play a multiplayer game if you are looking for a challenge with other people. At the present state it is a good start for Gal Civ 3. Many of the improvements over Gal Civ 2 are subtle and you don't realize that they are new to the game unless you go back to Gal Civ 2 and go... ohh that wasn't there before... Since it is a seamless transition for many things.
If you need convincing for the game, well I'll let others do that for you. The shipyard (the place to build and design your own custom ships) is a great place to hang out in the game right now until the AI becomes more of a challenge.
