"Practically everything a laptop can do"
Well not quite everything as well... An iPad is great for content consumption, watching videos, playing games or listening to music for example. It's not so hot for content creation, I wouldn't like to put together a big document or presentation on one. You can do this but the interface isn't really up to "heavy" use, Apple themselves admit that the design goal of the iPad's word processor (Pages) and slide show package (key note) were to let it tweak existing documents or to create simple new ones, they don't think of it as being a complete laptop replacement.
This may or may not be much of an issue. Also at present you need a computer to sync an iPad with, so you're going to have a "proper" computer around for that "heavy" work anyway. Though that said Apple are going to do away with that requirement...
"Besides which you'd look a bit of a tosser on a bus with one."
I see a lot of iPads on my daily commute now, lots of people are using them as big mobile devices for when they're on the go.