I think the most important difference between Alchemy and Django is the separation between the SQL abstraction layer and the ORM. There is no separation with django, making it very difficult to really get in there and make changes.
But I don't think this makes the django ORM bad. It is very easy for beginners to get up and running. It is very easy to understand the models and the query language.
As an FYI - 1.8 should make it possible to use .annotate() for constructs that aren't aggregates - like functions and extra select columns. You'll also be able to write more complex aggregates. It should give you greater control over your SQL, removing the need for .extra() in most cases (but not .raw()). It's what I've been working on recently: https://github.com/django/django/pull/2496
But I don't think this makes the django ORM bad. It is very easy for beginners to get up and running. It is very easy to understand the models and the query language.
As an FYI - 1.8 should make it possible to use .annotate() for constructs that aren't aggregates - like functions and extra select columns. You'll also be able to write more complex aggregates. It should give you greater control over your SQL, removing the need for .extra() in most cases (but not .raw()). It's what I've been working on recently: https://github.com/django/django/pull/2496