these two bug me a bit..
Windows can be even more functional than the other big OSes. but at the same time it can be less. Depending on your needs. (Unless you go 64 bit.. I can't find anymore annoying thing as running a game that won't install because the installer is still 16 bit)
I guess it depends on the definition and depth of complexity. Windows is designed primarily as a desktop OS, and primarily for the average-Joe user. As such, there are features it lacks and some aspects of its design that are weak, but only in contrast to the user's needs. You can do a lot of more interesting technical things out of the box with just about any Linux or Unix distribution than you can do with Windows, but whether it matters or not is ultimately up to what the user perceives as an important need or not.
The second one also depends on what "advanced" level is.. but at the same time, it is not really available to be tinkered with inside a code system, you can still do a LOT of modding to Windows; especially if you turn off it's fail-safes. (something I wished my Kubuntu box had more of )
Yes, Windows can be modified quite a bit - I certainly do it to my own copies of Windows and always have (I love stripping them down to absolute bare-bones minimums for efficiency, for example). The difference comes down to the level to which you can do these modifications. With Windows, you can only do so much before you're going to have to hack it or concede defeat. With most of the Linux and Unix OS's, you can change anything (even code your own additions or fixes if you really know what you're doing) so long as you have the patience and the desire to learn how. Doing so in a non-Windows OS though isn't always as simple and can be more tedious. Again - comes down to what the user is comfortable with.