[chongli]

A huge difference. Interactively, you can both ask and answer questions and you can steer the dialogue toward the pupil's areas of ignorance.

As a form of writing, you're left to guess at what your pupil might be thinking. Apart from Plato, few people have ever managed to succeed at this.

[abecedarius]

Plato succeeded if you judge by his historical rep, but I can't stand the guy, for reasons not entirely unlike the complaint starting this thread. Though good dialogs do exist (I like Raymond Smullyan's).

[nercht12]

Plato didn't imagine his audience to be idiots. He really debated with himself or with questions he'd probably heard other people actually make (since he was with lots of other philosophers). He's harder to appreciate if you don't think like him or his companions and thus don't follow his line of thought.

In a way though, Plato tries to tell a story rather than blabber away facts. It allows for the introduction of stupid or "less-intelligent" questions rather than setting up obvious straw men.

.02