For me it's the "one-more-turn" feeling that defines a superior game over a good game. Now as Slick mentioned this feeling is created by the rate at which you get things. Equally important is how significant these things are. I think this is where space empires makes a difference.
Compare it to civilization. Roughly that game creates its one-more-turn feeling with 2 types of "waiting". First of all it's the relative long term projects of research. Second it's the short term projects of building units. Units in civ are simple and linear in a way, the more high-tech a unit is, the stronger it is. Allthough there are a few things to spice this up (like flight and nuclear weapons and the resources aspect) it is my feeling that civ comes down to that in the end (don't get me wrong, I'm a great fan
).
For a game like Space Empires it's not that simple.
First of all it allows players to design units. Units can vary in size and strength from a simple escort to that full-blown defensive starbase that turns a warp point into an impenetrable wall for enemy ships. Thus waiting for a new unit can be anything from a short process(for a cannon fodder destroyer) or very long(for a game-balance-changing new baseship). Furthermore even the very powerfull, expensive, "takes-long-to-build" baseship has weaknesses. It's not allways a case of the bigger, the better.
This is because research is much less linear in space empires. Choice being the key word here. You can actually chose which techs you are going to develop and in what order. A lot of other games offer you limited choice (You have some choice, but the big milestones have to be researched in a certain order). Again compare to civilization, you can't race towards electricity while you are still fighting with cavemen. An effective ship design(being the offspring of good research CHOICES and maybe even a clever combat tactic you have developed in your mind) can totally shift the game's balance. This makes the "one-more-turn-waiting" to finish building that newly designed ship, or to finish that cleverly chosen research project just a lot more interesting.
What also adds to the feeling of "one-more-turn" is the sheer magnitude of some projects. Just take the simple invention of "Ice planet colonization", it can really can double your growth potential. Take dyson spheres, technology to blow up stars, create black holes, create planets or technology to board, take over and analyze a high-tech ship from your opponent. There are numerous other examples and they have one thing in common, they can alter the gameplay environment and the balance in ways I have never seen before in this kind of game. Logically it makes waiting to finish that project much more interesting.