There are lots of things I want to do with email that I can't do with SMS. Maybe half the emails I send could get across most of what they need to in 160 characters. Other recent messages included detailed tech support/troubleshooting, a list of equipment and design discussion with a partner about an application we're developing.
None of those would fit in 160 characters. There isn't even a way to losslessly compress them in to 160 characters. I can always write a short message when I only have a little bit to say though.
I also dislike twitter and like RSS. I'm evidently not in the majority.
The only functionality that SMS provides and email doesn't (in most countries) is network notification, which can be easily fixed. It would be relatively easy to replace SMS with email without users noticing the difference.
