First off, I can't change the fonts and styles (like Heading 1, Heading 2, etc) to the way I like them. And, I can't select my own font from my system, period. I only have a list of 6-7 fonts to choose from. (I use "Calibri, Tahoma, Arial" as my default font for just about everything..)
Second, There is no way to upload nor work on files other than basic RTF/HTM documents and spreadsheets. For instance, if I'm going to use this system exclusively, how do I create posters, flyers, business cards, forms, etc, as I would with Microsoft Publisher? How about OneNote?
Only supporting basic documents and spreadsheets goes further to cause more problems. I generally keep all kinds of files mixed in with my documents. I keep (other people's) PDF files, images, links to websites in .url files, raw .html files, XML, source code files, and many more. Where would I put these files if I started using Google's web-based services?
I love (and live by) the "drop everything into one bin and use search to find it" mentality, but what use it is if I can't drop everything into it? Google Desktop Search allows me to do just that right now, without having to be connected to the Internet.
Which brings me to the biggest reason why I won't use it. There isn't any way (that I can see) to backup my files to my desktop for when I'm disconnected. Is there? What about when I'm traveling? How about when I purposely disconnect from the Web so I can get some writing done or when I go to the park to work while the kids are playing? (And, no, getting Internet access via Cingular isn't an option for me!)