I want to preface this by saying that Django isn't bad. It's really good. However, there are some maddening things.
For example:
In Rails, you can say
Article.find(:all, :include => :comments)
and that will get you the articles with their comments. In Django, you can say
for a in Article.objects.all():
a.comments #hits the database again! argh!
Before you say, "Django has select_related() which does the same thing as Rails!" select_related() doesn't work in any useful way. select_related() only works on the side that defines the foreign key - so you can get the single article that a comment belongs to, but you can't get the comments on an article without running another query. Yeah, that simple, basic, every ORM should support it case isn't handled. And it's maddening.
Forms handling and validation is another sore spot. Some validation happens in the model, some in the form, ModelForms does somethings, but not others, editing related objects is a mess. In some ways it's shocking that it's such a mess considering how awesome the admin interface is. However, I always find myself manually dealing with forms to a point that's just annoying.
The lack of a true "if" in the templates. It's just limiting in a way that I don't find useful and I find that it makes my code less clear by nesting things in weird ways to get the desired effect.
Stagnation. Django is awesome, but sometimes it takes forever for small things to go anywhere because there's a genuine hesitance in the community to "dictators". By dictators, I mean personalities like DHH who say, "well, this way works and is elegant and we're going to go with it." On the one hand, it's what makes Django really attractive as a community. Rails attracts the know-it-all personalities. Maybe DHH and Zed do know it all, but most Rails programmers definitely don't. People in the Django community are really helpful, respectful of other's knowledge, and love learning from each other. However, it also means that even tiny features must be rehashed over and over, filed in triplicate, lost, found, stamped, checked by a committee of 8, rewritten from scratch, named new-X or X-refactor, break API compatibility, and finally get into SVN. Ok, most of that was a joke, but there are lots of little areas where it just seems like Rails is willing to move on something based on the general mood where Django wants a more formal assessment and wait period before deciding whether something is ok to be in or not. And for all people complain about DHH's one-way-to-do-it mentality and Django's more open structure, Rails isn't so closed and Django not so open.
Django's big selling points (for me) are that there's a ton of python out there that's beautiful, reliable code and the admin interface that allows you to deal with content neatly and easily. Clearly there are other selling points like MVC and ORM and whatnot, but there are many frameworks that have that. Django is a lot easier to understand (at least for me) in its implementation. Django's documentation is top notch which is amazing compared to other projects, but Rails is catching up with their guides and screencasts. Django also tends to document things like their file backend hooks better which makes creating plugin-type things easier, but again Rails (with the help of the Merb team) is going to be a ton better in this area.
You won't go wrong with Django. The only fatal flaw is really the first one and it's just unacceptable at this point in time. But it's a great set of code with a wonderful community. I don't think there's another framework I'd prefer. Maybe Rails, but nothing else.