I just went ahead and did it. It was actually fairly painless. I have to re-do my theme, but everything should be working.
Please post if you see something that’s hideously broken.
The Drupal release of 4.20 has all the details of what has changed. It should be much easier to do a full CSS theme, plus – friendly URLs!
Should increase the Googlejuice. I have to add some lines to the .htaccess file to get my most popular item working. Or rather, it’s old url.
I guess the other steps will include looking what sort of modules have been updated, and perhaps install some new ones – like smilies :p