Regency=empire gowns, strict society, the fun of sneaking around those strict mores, and Napoleon. Every Regency romance I've ever read either vaguely mentions the Napoleonic Wars or skims right over the fact that Napoleon controlled 3/4 of Europe and travel wasn't as free as it once was.
I'm guilt of skimming over it, despite the fact that many of my heroes were once in the British Army, I don't tend to use a lot of detail about the actual battles. I'd love to one day write a series set in Portugal during the wars with a romance developing over several books while fighting the French, but then I watched every single Sharpe movie obsessively, read the books, and stalked the Sharpe Appreciation Society.
So the question is, what fascinates us with the Napoleonic Wars to the point that we know about them, research them, but barely use them?