9 Things that DHH likes about Rails 2.0

Breakpoints are back. Wait, actually there's a real debugger in there?!
HTTP Performance has improved. Any JavaScript and CSS that uses the *_include_tag with :cache => true will be bundled and gzipped up, so the browser will make less connections and fetch less data across the wire. Also spreading assets around to different servers will be easier to parallelize the browser downloads.
ActiveRecord Query caching. If there's no inserts, updates, or deletes (which would invalidate the cache) it will just return the next selects out of its cache. You use ActiveRecord::Base.cache do/end, though I guess this will be the default soon.
Splitting out rendering and response type for template names. This adds conventions for the template names which help Rails figure out which one to use for a given response.
config/environment.rb is now modularized
DRYer migrations - fields are now grouped by type (I guess we lose the order, but don't think that really mattered.)
HTTP Authentication is baked in now. You can use authenticate_or_request_with_http_basic in a before_filter. (Also just authenticate_with_http_basic and request_with_http_basic.)
Assume MIT license.
Spring cleaning. I think the Rails Core actually means it when they deprecate things (as opposed to Java which sort of just warns you forever.)