The biggest reason I love linode now is the control panel. Slicehost also had a good control panel, but the Rackspace Cloud CP is severely lacking. It's extremely slow and the DNS is in a really awkward spot (click a server and then click DNS to see a global DNS...it doesn't make sense).
I was a fairly happy Rackspace Cloud customer but Linode is just a great experience. What is Rackspace doing to improve the cloud offering?