Interface, interface, interface. Make it easy to do things, and give plenty of in-game tooltip and help options.
Bind it with history. Give us details (in-game, in a separate database etc) about units and leaders. Engage us!
Make it easy to mod. Please...
Love the looks, hope the engine works as well.
We're planning to make a Flash presentation that will be a walk-through of the game features. It will double as an advertising tool as well as a tutorial. We have tooltips for many of the controls and stat-bars in the game. Some things just can't easily be thoroughly explained in-game, such as the economic and combat rules.
In our in-house testing sessions we've found that some testers prefer to ignore most of the economic rules, whereas others (are you reading this Mr Z.?) have fun endlessly tinkering with trade proposals and labor allocations. The advanced economic stuff can easily be ignored if all players want to do is build units and fight battles -- for the most part, your economy doesn't fall to pieces if you ignore it (you may have to adjust the tax rating or take out loans).
Some aspects of the game will be easy to modify. Basic unit attributes, costs, starting locations, provincial production values, some global game parameters, attributes of generals -- all can easily be modified by changing the plain text data files. (We are talking about having monthly tutorial-type newsletters on our website describing how to mod various aspects of the game.) The scenarios are just a directory with certain data files in them, so players will be able to create their own scenarios -- though this will require some small amount of instruction.
As for the engine working, we are able to play long single player games all the way through with no critical issues. I just played a 12 hour game over the weekend with no serious problems. There may be some issues on some operating systems that we don't normally test on -- I'm getting unusual bug reports from one beta tester who is running Windows 2000. Multiplayer has some critical issues to deal with, but most of these are all at the "edges" of normal play: things like recognizing when a player has dropped and smoothly dropping a player, and server setting game options for players.
Thanks for your interest.