Converting Django project from MySQL to Mongo, any major pitfalls?
I have recently tried this (although without Mongoengine).  There are a huge number of pitfalls, IMHO:

No admin interface. 
No Auth  django.contrib.auth relies on the DB interface. 
Many things rely on django.contrib.auth.User.  For example, the RequestContext class.  This is a huge hindrance.
No Registration (Relies on the DB interface and django.contrib.auth) 

Basically, search through the django interface for references to django.contrib.auth and you'll see how many things will be broken.
That said, it's possible that MongoEngine provides some support to replace/augment django.contrib.auth with something better, but there are so many things that depend on it that it's hard to say how you'd monkey patch something that much.
I want to try Mongodb w/ mongoengine.  I'm new to Django and databases and I'm having a fit with Foreign Keys, Joins, Circular Imports (you name it).  I know I could eventually work through these issues but Mongo just seems like a simpler solution for what I am doing.  My question is I'm using a lot of pluggable apps (Imagekit, Haystack, Registration, etc) and wanted to know if these apps will continue to work if I make the switch. Are there any known headaches that I will encounter, if so I might just keep banging my head with MySQL.