squirrel, you don't quite right in some details.
1. MySql really fits to YAF it can offer all features required. But there'are multiple problems. In some cases perfomance sucks because of complex queries, MySql is really a very primitive database, in some I use really tricky technics. It's unbelievable, but even Firebird fits fine with YAF:evil: .
2. Everyone who wants to use MySQL with a YAF legacy version can find
YAF v.5337 Simply find the changeset here
http://yafnet.codeplex.com/SourceControl/list/changesets and fit it together. But I will not support it or will make it for money. Unfortunatelly it cannot not be cheap.
3. I will not support MySql for newer YAF versions by various reasons. The most important is that it requires some special technics to get perfomance for large forums. By all the reasons there's a fork now called VZF on github. It will be supported by me in future.
4. YAF has ORMLight to access data in 2.0 RTW, everyone has option to write his own scripts.
http://www.nearforums.com/ is a MVC3 forum that supports MySql (at least it's declared).
It could be an option for small forums. But it's much more slow then YAF with mysql when it comes to scalability.