A case study dealing with scaling would probably have to include questions such as:
- how many posts / pages ?
- how many comments ?
- how many registered members ?
- how much traffic ?
- performance related plugins used ?
- server setup (like, basic shared stuff or beefy dedicated one)

On the feature topics, I'd ask:
- existence of an interface with something external to WP (like a forum, phpBB, a gallery, etc...) and if so, how is this done (plugin, dirty hack, etc)

I'll post more if I think about something else