I'm very impressed with WordPress as a piece of software in general, and particularly at how it's managed to implement a text editor inside a web browser. However, as a text editor it's kind of annoying. It doesn't work terribly well in Internet Explorer for one thing. Even on Firefox and Chrome, it suffers from "one scrolling window inside another" syndrome which is common on many complicated "web applications". Once you've got a got more than a screenful of text in the edit box, you get an inner scrollbar to scroll it up and down. Being a keyboard-oriented sort of person, I usually use the PgUp and PgDn keys for this. PgDn works fine on the inner edit box until you get to the bottom, at which point it stops swallowing the PgDn keypresses and passes them on to the outer window, which then scrolls the top of the edit box off the window. I can't undo this with PgUp because that goes to the inner edit box! This behavior breaks the idea that PgUp and PgDn should do opposite things.
One way to fix this would be to make the edit box expand to fit the text instead of scrolling. There's probably a plugin for this (I've been able to solve just about every other WordPress problem I've had by installing a plugin) but I have no idea how to find it - the obvious searches didn't help. So most of the time I write blog posts in my preferred editor and paste them in. However, I still end up doing a pass over the text in WordPress to fix up the line breaks.