The rumors were true. Age of Empires II HD (which includes the content from the Conquerors expansion) is coming to Steam in April. And it's been aged up to the Modern Era (courtesy of Hidden Path Entertainment), with a host of tweaks and new features. We've got more details and a graphical comparison below.
For my money, AoE2 was already one of the nicer-looking games of the 2D isometric era. With the HD release, a fresh coat of paint and a vastly expanded resolution will make it look even nicer. You can almost see the contemplative expression on the face of a knight undergoing conversion. "Wololo, you say? Well... that really does make sense, now that I think about it. Red team for life!"
Here's an example of the original game, running at the native 800x600 resolution:
And here is Age 2 HD, cruising along at a respectable 1920x1080:
We're also getting Steam Workshop (because, let's face it, making custom maps based on Back to the Future was half of the fun of Age 2), multi-monitor support, and a small army of other technical improvements. And it's only going to cost you $20.