The biggest Chromebook advantage, in my opinion, is that it never slows down because you never change the software in any meaningful way. For the average person who bloats their computer over time until it isn't usable, that is huge.
Unfortunately adware/malware Chrome extensions already exist. Granted removing that is much easier than removing some nasty software installed on a traditional OS, but the type of user that's not savvy enough to avoid malware wouldn't know how to remove a Chrome extension :\
