I recently upgraded to Wordpress 2.1 and switched to K2 from the plain old vanilla Kubrick theme. K2 is an amazing it allowed me to remove some modules because of it's awesome "sidebar modules" features. You'll just have to try it out to see what I am talking about. It's also given me AJAX-commenting and AJAX live search. Also live archives. I recommend trying out all these features. I also stopped using Jerome's Keywords module and am now just using Wordpress' built-in categories. I was tired of always wondering if a module like Jerome's Keywords was going to work if I upgraded Wordpress. Now I have no modules that use the database besides K2 Means I can usually update to other Wordpress version without much hassle.

Techie stuff:
I track Wordpress upstream sources in my Subversion repository and then merge-in upstream changes into my trunk. If any of you geeks are interested in making your upgrades as painless as possible, that is the way to go. Far easier than Wordpress' upgrade instructions on their website. Not only that but I can hack any part of the wordpress core if I wanted to and still get upgrades.