Wherever I Go, There I Am
Connectivity obviously matters with Web apps. Long flights and train rides are likely to separate you from your Web-hosted data (not an issue for me, as I bike between work and home).
Even this problem could vanish in the near future, however. Google's engineers are perfecting an offline synchronization plug-in, Google Gears. But to this point only Google's RSS feed reader is fully compatible with Google Gears.
Via Google Gears, Zoho Writer offers partial compatibility, permitting you to cache and view read-only versions of documents while offline. But by the time you read this, the company may offer full offline synchronization.
Another major concern about online apps: What happens if a natural disaster or server outage wipes out my data? Google says that it backs up data files almost as often as users change them, and Zoho's official response is "do not worry." But to be cautious, individuals should download and archive their key documents regularly--another reason why high-speed access is vital if you exchange desktop for Web apps.
Privacy concerns may scare off some people. You have to trust a third party to protect your unencrypted e-mail and other data on their servers (see "Is Google Too Big?").
But for me, the convenience outweighs the risk that Google will fumble the ball on security. I like being only an Internet hookup and a mouse click away from my documents on the Web.