My question when it comes to Wordpress and scalability is: How good does MySQL scale?
As far as I know it can work, but there are much better DB solutions. Being able to use a different database then mysql in wordpress would be awesome.

Currently mysql is a big point in what keeps me away from using Wordpress to build a large scale service with a lot of users and user interaction.
Of course it could work, but I wonder why I should spend the effort to make it work when we could just use an alternative that scales better, faster and is easier to handle - even if that means building things from scratch (mostly).

I would love to hear thoughts on this, as I love WP but I don't see much use of it besides the niche BP community and online newspapers.

And yes I agree with Anh Tran, practical tips would be much better than theoretical talk.