What's wrong with this Apache rewrite rule?

Here's my .htaccess:

RewriteEngine On
RewriteRule ^([^/]+)$ /index.php?id=$1 [L]

And requests keep causing a 500.

You are entering rewrite loop. Change it like this:

RewriteEngine On

RewriteCond %{REQUEST_URI} !^/index\.php [NC]
RewriteRule ^([^/]+)$ /index.php?id=$1 [QSA,L]

or like this (should have the same effect behind):

RewriteEngine On

RewriteCond %{REQUEST_FILENAME} !-f
RewriteRule ^([^/]+)$ /index.php?id=$1 [QSA,L]