1) My new neighbor recently had a motherboard die on him. Because it was an OEM computer (HP, I think), replacing the motherboard with the exact same would have been prohibitively expensive, and replacing the motherboard with something else would have required a new license. (I know you can sometimes talk MS's CSRs into letting you reactivate, but 'officially' it's not allowed)
2) I recently bought a copy of Windows XP Home Upgrade. I didn't buy it at a store. In fact, I bought it from someone else who had. This same person had intended to install it on their computer running Windows 2000. And they couldn't. Not because of any technical reason, but because of arbitrary limitations imposed by Microsoft. See, in order to upgrade, you have to have an existing installation of Win95/98/98se/ME, OR an install disk for Win95/98/98se/ME/2000. And they fell neatly into a corner case. And they couldn't return it, because Best Buy won't take back opened software (can't blame them, really).
3) The gazillion versions. Yeah, lots of other people have complained about it, too, I know. But you know what? It's even worse than you typically hear. Take a look at all of newegg's offerings for Windows Vista. There are twenty-seven. Not only do you have the Basic/Premium/Ultimate/Business versions, you have a choice between Retail and OEM versions, Upgrade and Full versions, 32- and 64-bit versions, and single- and 3-packs. And, as we see in #2, if you get the wrong one, you're screwed.
4) The price is ridiculous. Period. Windows, in and of itself, doesn't help me do things. It's the applications that count. Comparisons to a Linux install (cost vs. what you get) are apt here.
5) I work for a big, faceless corporation. Our IT people buy (or do they lease? I don't know) laptops from Dell. They pay for the license that comes with the laptop, then pay again through their site license. Yep, Microsoft is double-dipping. I bought my own hard drive and happily installed XP using that unused license (and dual-boot Ubuntu), in case I actually want to do something useful with the laptop, which is otherwise tightly locked down. Off the company network, naturally.
6) It's becoming pretty difficult to purchase a computer from any of the big OEMs without getting burdened with Vista. I get the feeling that the number of people who prefer Vista over XP is overwhelmingly small. Yet Microsoft leverage their market position to force a copy of Vista onto nearly all new computers. And if you want to switch back to XP, you get to pay Microsoft again.