I would go for Python + Django. Avoid Google App Engine. There are big gotchas with using Django and GAE. You essentially have to throw out the Django ORM because Google places restrictions on what SQL queries you can use (for scalability reasons, none of which are your concern for an MVP or even a moderately sized startup). You can avoid this using Google Cloud SQL, but then you have to use MySQL. PostgreSQL is recommended by most of the Django community. You are better off using something like Heroku (which has great documentation). You can either use Twitter Bootstrap (maybe tweak the defaults with something like Jetstrap), or use a template off themeforest or woothemes. If you use a theme, you'll have to tweak the HTML to make them work with Django's templating.
The problem with Wordpress is that unless you're doing a plain vanilla ecommerce site or some other limited choices, there are many kinds of MVPs you would not be able to build with it. Not to mention that you would be rewriting everything eventually if you use Wordpress.
Using meteor and node.js cover different product use cases (real-time apps or apps with a lot of connections). So it depends on your product. Keep in mind that meteor is not mature yet.