There are usually two scenarios of registration. Registration as direct action (user wants to sign in to use service) or registration as required action (user wants to add comment as in Anna's post).
For the second case of indirect registration it is better to return user to the context, where he've met the requirement to be registered to do certain actions, as other commentors told.
But if registration was a direct action, it can be better to send him to some kind of welcome page which tells him what to do and/or what he can do next. It can be a special page, that will appear once, or for some services it can be a casual dashboard that the user will often use in future but with additional extra messages for novices (for instance, link to tour, hints, instructions, even some master to fill in necessary information etc.). This can help make the process of immersion to new context increasingly easier for user — particularly for complex services (such as professional social networks, intranets, blogging platforms, prototyping tools and so on).