Perhaps aesthetic - both Windows 1.0 and 2.0 were (to me at least) very ugly. Things got a bit better with Windows 3.0 and 3.1 (and easier to program) but it wasn't really until Windows 95 that the whole thing came together. One thing you have to give Microsoft (at least back then) is that they did keep trying. And, speaking as a Windows developer, their documentation was very good.