I'm surprised you didn't mentioned the "sxs" folder, that would have been what I consider a valid point since they don't tell you before installing that this folder is going to eat your free space on the system partition as a everhungry monster, with little to no counter options to get around that
It's basically a compatibility feature found in XP. I think Vista/7 do something a bit differently? Not sure.
One of the big, big issues with older OSes is something called "dll hell." In the early days, when the system files weren't so bolted down, you could modify them - and every app in the world decided to do so. That created a lot of issues, especially when more than one application would overwrite the same file, or even replace a newer file with an older one.
It was, as the name indicates, hell for the user. A major source of a lot of bluescreens and crashes.
I think that folder is Microsoft's answer to the problem: Instead of overwriting the file, create a copy of it in the sxs folder and fool the software into thinking it was writing to a system file when it really wasn't.
Strange it should get so big, though.
The folder that usually gets big for me is "System Volume Information" which includes the "system restore" feature of Windows. Allows you to roll back the state of the OS and retrieve old versions of files.