a) Past statuses get archived. Each of them gets a permalink. You get RSS on them. There's a huge difference between just seeing what someone's doing right now, and being able to see a concise summary of their day or days laid out before you.
b) You can access it via mobile phones.
c) It is a social network. You can discover interesting people by clicking around.
d) You can follow others. This one is so huge I'm certain I still haven't grasped all of its implications. Twitter is a social network built on a single simple one-way connection, and already there's more value flowing through it than through facebook.
When I go to facebook I get spammed and poked and bitten and humped and god knows what else. Facebook is made for senders. When I go to twitter I can get a sense of what my close friends and family are up to, no matter where on earth they are, so that when I do open up my IM window to chat with them I don't have to start with the tedious "how was your day?" I have prebuilt objects to begin a conversation with. Twitter is built for recipients.
twitter scales more at the edges.
Forget mode, protocols, technology. I can be mobile with a phone, send a message not only to another phone but as data that can be further processed by machine or other people.
Longer text is better for some writers - they don't have to work to fit in their message. But it's better for all readers - they can come read their twitters knowing no single entry will swamp it.
For me, I don't think it's an issue of who's a geek and who's not, it's more "who's going to leave their AIM or ICQ protocols to use something on the web that does the exact same thing?".