The differences with standard Drupal 7 are scattered about the Pressflow wiki though not documented very well, perhaps because Pressflow 7 is relatively new. (There is a comparison chart that has not been updated for Pressflow 7.)
Although it is sometimes said that Pressflow is a drag-and-drop replacement for Drupal because you usually don't have to make many changes to your modules' configuration, to get the most out of it you'll want to set up Varnish, APC, Memcached, etc. This will probably be impossible to do on a shared host; you'll need some sort of dedicated server and SSH access to go this route, and it's unnecessary to do so unless you actually have a high performance site.
If, however, like me you just like trying out new things, you could sign up for a free account with Pantheon (disclaimer: I'm a member), and try out Drupal on their servers. Pressflow, Varnish, etc. all come pre-configured. Pantheon was founded by David Strauss, who also co-founded Four Kitchens, the company behind Pressflow.