I disagree. It's pretty obvious that though a great number of changes came through with Rails 3, one of the goals was to have a short uptake path. This is readily observed through the decision to maintain Prototype as the default JS library, ERB, and others, despite the gross popularity of the alternatives.

Introducing Arel, and improving ActiveRecord in general, serve to bolster this attitude and approach. That is, to improve the state of the defaults, provide an easy path for uptake, whilst now offering the alternatives cleaner means of tying into the Rails system.

To address your side nodes...

#1. A hat tip would have been nice, but I don't think too many folks were truly upset in the long run.

#2. Meh. The only thing I really miss from DM is the declaration of attributes. Arel is pretty tight. Check out Mongoid. It's pretty slick.

#3. Ex-Merb user too. AR does the trick sometimes, but I'm happy to have the freedom to use Mongoid, Sequel, etc.