I've played 2 games for about 10 game hours in large single system random (Varsari) and huge multisystem (Advent)
* Starbases - I like them, and I think their offense and special abilities are about right, but defensively they seem too weak, even when completely upgraded. Intuitively smalish fleets have what seems to be too easy a time taking them out too quickly (and without much loss if you micromange). The base can protect your other upgrades but isn't really much of a bullwark (aside from the fact that ). Perhaps just increasing the armor several points of the TEC/Advent bases would suffice (maybe not the Varsari as relative vulnerability in exchange for being able to move is appropriate). How about tractor beams to nab some of the frigates so they can't escape? 
Mines: difficult to lay in a real-time effective way (as in a ring around the planet or your facilities or the jump points. They seem mostly annoying and their presence never altered my strategy much. It wasn't worth the time to lay them, or the clutter on the screen to have them so I stopped using them myself. I did send out some scouts to clusters in between my fleet and what I wanted blown up and let strike craft take out the mines, which occasionally slowed me down when there were too many of them. I like the concept but they need to be reworked. Scouts should have an "auto-attack-mines" button that has them seek out and destroy the mines.
System Resources: I have a E8500/ 4GB/ 32bit Win32 ; I turned all the 3D settings down to low. Near midgame gametime seemed to be taking about 2 seconds of realtime. Entrenchment makes a lot more sense on big maps with lots of opponents ... a system like mine should be able to manage Huge random, 10 players with 1:1 game/realtime.
Cost structure: A nice thing about Sins is that resources are %tax on produciton rather than absolute cost. Still, it might make sense to have Starbases and/or mines take up some capacity. Perhaps a logistic upgrade path that taxed income that alllowed some capacity of static upgrades. That way you could pay to have a few planets really really difficult for a big fleet to take out at the expense of income and/or thinner defenses elsewhere.
