better support for non-relational databases
Rails ActiveRecord make easy to work with a relational SQL based database but there's a few support for the non-relational ones, like a document-oriented database. In many cases the non-relationals databases are a better choice.
Matthew Higgins commented
the api for a non-relational database should be different than a relational database, because the abilities of each are different. Why hide away the goodness of a relational database? I would consider a dedicated 'hash' based storage for memcached, tokyo cabinet, etc.
ORM agnosticism is useful!
I never could fully swallow the everything-is-centered-around-the-table-record-ness:
One of the goals with Rails 3 is being ORM agnostic. There's work on documenting what ORMs need to be compatible with rails, and providing assistance for common libraries like Sequel and DataMapper to be completely compatible.
from your mouth to god's ears. in this case, perhaps wycat's ears.
i couldn't agree more. for me, the dbm family, especially tokyo cabinet, is almost always the best tool for the job.