Why? How many reasons do you want?
Here's the stuff I like more...
- Does a better job with HTML messages
- Better folder layout
- 3 column view is teh ****
- Grouping mail is a godsend
- I like the built-in calendar better than iCal
- Linked items
- Notes and task lists
- Doesn't automatically download pictures (faster)
- Redirecting messages
When I recommend a Microsoft product, you know it's good.