* Faster to launch
* Faster to load pages
That is a good start.
Reader takes stories on some pages and strips away all of the junk around them. Want to see an example?
Open [this ESPN page in Safari 5](http://sports.espn.go.com/mlb/news/story?id=5262731) and then click Reader in the address bar (or press ⌘ SHIFT R). You can even email this lightweight version of the story. Very nice.
Add in history link from the top sites page and other internal improvements and Safari 5 is quite the nice upgrade indeed.