WSN Knowledge Base requires PHP 4.3.0+, but uses less memory with PHP 5. It's recommended that you choose a PHP 5 host if you can. It works with MySQL 3.2+, but has better performance with MySQL 4.1 or higher, which is also required for the similar articles and fulltext searching options. In order to take advantage of URL rewriting you need an Apache webserver, which basically means any non-Windows web host. If you're interested in converting attachments to FLV, Linux is the recommended OS due to the developer's familiarity with installing ffmpeg on it (but feel free to use whatever operating system you're most comfortable with).

Try to make sure your host has one of the following:
1) PHP's ftp extension
2) suExec
3) PHP running as CGI

If you have any of those, permissions will be handled automatically for you. If you don't have any of them, you'll have to spend a lot of time setting permissions manually. If you don't know how to find whether you have one of these, just email your host to ask.

The GD library is also important to have for the security image and thumbnails, though virtually everyone will have it since it's bundled with PHP by default these days. If possible, the bundled version of GD is better than the external version since it makes the image_rotate function available.

A few hosts disable both fopen url wrappers and curl, effectively cutting off your server so it can't download files from elsewhere. This will make things harder for you by disabling the automatic upgrade.

A servertest.php utility is available on request which you can use to verify that your host is ideally suited.