I think the best thing will be the fact that everyone will have access to iMessage. None of my friends have it at the moment, so I've never used it.
What I love most about iOS 5 is custom SMS ringtones.
I hate when someones iPhone gets a message and I proudly take out my cell, thinking I'm the important guy.
Wirelessly posted (Mozilla/5.0 (iPod; U; CPU iPhone OS 4_2_1 like Mac OS X; en-us) AppleWebKit/533.17.9 (KHTML, like Gecko) Version/5.0.2 Mobile/8C148 Safari/6533.18.5)
One great feature is that you no longer have to have autocorrect on with misspellings, it is so annoying when you type something correct, and it gives you something you didn't want instead.