As expected, I lost track of time and wound up playing Galactic Civilizations II all night. There's just something about sorting through menus and trying to figure out my optimal build order that I find very relaxing. It took me awhile to get back into the swing of things, but this isn't actually the most complex strategy game I've played. It's got some subtleties that can make it a little tricky (for example, it's a good idea to run your economy in the red whenever possible in order to maximize your population growth, and thus get more tax revenue in the long run), but it's more or less a paradigmatic 4X. You've got the production/research/currency triad, tile-based settlement improvements, progressive tech trees, and a straightforward diplomacy system that mostly just exists so you can exploit the AI with technology trades and manage the order in which you fight your rival empires.
The most distinctive aspect of the game is that there is literally no downside to expanding your empire. More planets is always better. And while that's generally, de facto, the case in virtually every 4X game, it's actually rare for there to be no expansion penalty whatsoever. This can make larger maps a real chore to play. You've got to keep grabbing territory, lest your opponents get an insurmountable lead in planets, and thus wealth, technology, and military power. It also means that you've got a lot of micro-managing to do, even in the mid-to-late game.
On the one hand, I like this, because deciding on which tile improvements to build is, like, my favorite part of the genre. On the other hand, this sort of endless expansion treadmill is kind of stressful. My instinct is to look inward once my borders are secured, but if "secure" isn't good enough, that means I always have to be looking outward.
The upside is that it's really simple (if not always easy) to start snowballing in power. Because territory is so intimately tied up with power, once you've got a plurality of the map, there's no force that can stop you (unless the AIs band together, but that's pretty rare).
I've only got eight hours left, which should be enough for two or three more games on a small map. My plan is to reacquaint myself with the expansion packs asymmetrical tech trees. I don't remember them having a huge effect on gameplay, but I do seem to recall that they added a few interesting wrinkles to the standard game progression. I expect that I'll reach 20 hours by this time tomorrow.