The game can have up to 10 players, has a huge tech tree, very flexible and detailed ship design, competent AI, strategic or tactical combat, play-by-email support, and a large amount of freedom to customize the game and victory conditions to your liking. There are a lot of fun subtleties and twists they've put into the standard stellar empire fare, such as advanced technologies that allow you to create wormholes and suns. It also has an online tutorial, and the interface is pretty good - allowing you to access lots of data without getting overwhelmed.
The only real downside is the graphics, which do their job well but are definitely of the simple and plain variety. On the other hand, the game has such a big fan base that many players have made their own ship graphics sets which you can download and use, so you can have Star Wars craft vs. Star Trek craft if you like! Space Empires IV is available as a demo, and its official release is scheduled for Nov. 2000. It promises to improve the graphics of SE3 and have even more features.
If you can overlook the lack of flash and get to the rich game within (as any true old-school game lover should), you're in for a real treat with Space Empires. Check it out!