The problem with Twitter (yes, there is only one) is this:
As long as phone companies charge outrageous fees for text messaging (even paying the flat fee for unlimited messages is more expensive than it ought to be) Twitter will be unable to reach across platforms as seamlessly as it is intended to. A major piece of its functionality is too much of a luxury for most to afford. As it works right now, conscientious Twitterers (or those who have received their first phone bill since signing up) know to turn text updates for the cell phone off. In fact, I never use my phone to Twitter because I haven’t added a texting package and don’t feel like doing so until I get my iPhone (still trying to figure out how). That such a huge, integral feature of the application is restricted to those who can afford the exorbitant fees for texting reduces the chances Twitter has of becoming a huge success. Without the ability to cheaply (read: freely) use Twitter on the go, it is, in effect, no more than a minimally-featured microblogging platform or a slow group messaging client.
Twitter’s real value comes in its ability to reach across platforms and devices, but until someone forces the cellular phone companies to offer sensible data plans, there can be only one true answer to the question “What are you doing?”:
Sitting at the computer.