Most of the time they are nothing more than an site:xyz.com equivalent that is just easier to access and you can group them to say something like !dev and search hn, stackoverflow etc in one search without all the blog spam in between.

The engine should automatically detect that I want to be booted to a third-party site? Err, no. That would be infuriating behaviour if it happened without explicit instruction.

It should detect if a query is very likely to be something that appears on say stackoverflow and then offer it as a first search result.

Does doing the hashbangs actually throw you directly onto the site? I'd assumed it just restricted results to that site.