The fact that I'm forced to override font sizes for usability because sites don't respect my default settings is bad enough. When they break because of it, it becomes terrible design.
But forcing everything into the same font seems a bit excessive; I've actually seen a couple of sites make really nice use of fonts, and you can still install an extension that turns your page into an "easy reading" mode on a key press.
What do you do about fixed width vs proportional fonts?
You have to click Cmd-+ once in a while to zoom in on pages that are made by idiots.
I actually think this is the future of browsing (at least for people who do a lot of reading in-browser). Books that used non-standard fonts and colors would be laughed at (say, Papyrus on a beige background for body text), but just because it's easy to go wild with websites doesn't mean people should do it.