OH!

Here's another thing I love:

MAIL. You can mail an item/money to another player, or just mail a message for a measly 30 Copper. The downside is that you can't mail from Alliance to Horde, but that's story-related.

1 thing I hate, is that there is almost NO way to communicate from Horde to Alliance. You speak different languages (mostly), you can't whisper them AT ALL, and whatnot. You just can't do it. I understand for the Role Playing value of the game, but still...

I haven't seen much of the Guild system, so I can't really comment much on this.