I am needing to create a sort of "plug-in architecture" whereby different
Web sites I maintain on the same server have slightly different
functionality. Some Web sites will need to use the functionality in some
assemblies, while others will get similar functionalilty from different
assemblies.

What I was thinking about doing - and I'd apprecaite your feedback on this -
is to load the assemblies required by a particular site into the site's
default application domain, and do this loading during Application_Start. I
would load the assemblies via Reflection (e.g., Assembly.Load()).

Two benefits of doing it this way:
1. I can have all the logic in one place... this is the logic that
determines which assemblies to load for the current site.

2. A minor benefit is that when members from a given assembly are eventually
required, the assembly is already loaded, thereby helping with runtime
performance.

Are there any significant downsides to doing it this way, or reasons I
should not preload these assemblies? Any "gotchas" I should account for?

Thanks!