Grande Armée is uniquely different from most Napoleonic games in several ways. Two of these (flexible basing and no time/figure scales) have already been mentioned above. Some others include:

Variable Movement: You only know roughly how fast your units will move each turn. It's variable, based on your level of control of those units, the presence of the enemy, the weather, and the terrain. (Terrain penalties for movement are also unpredictable.) And once your units get in close with the enemy, they may or may not attack, in spite of your wishes. Does this sound complicated? It isn't; one six-sided die roll takes care of it all, for each unit.

No Written Orders: Using the Command Point (CP) chits, each player makes decisions about the control of his forces. Those which aren't controlled may or may not behave as he wishes.

Weather is extremely important, not an optional add-on at the end. Visibility is central to the game's command and control system, and ground condition affects everything.

Chaos: You're the army commander. So you can't control things like skirmishing, opportunity charges, evasions, and so on. All these things happen on their own, often in ways you didn't want or expect. If you expend your precious CP chits to micro-manage one portion of your army, control slips in the other parts. You can only do so much at once, and for the rest you hope for the best.

Army Morale and Pursuit: You have to conserve your cavalry for a pursuit (if you're victorious) or to cover your retreat. Unlike most games, where there's "no tomorrow," Grande Armée has a seamless procedure for campaigns and multi-day battles, so that one battle flows into another, and you have to make historical decisions about whether or not to risk committing your reserves. You might win the battle, but the enemy gets away because of his cavalry screen, or darkness, or bad weather, or....