I would take a look at Vanilla. Mainly it's a forums script, but it also has blogging features.
I has all the things you mentioned. Except is looks good enough:
- the entire code is about 500k
- it's well structured(MVC and well done), it supports extensions
- I'm not sure about tags(for sure they are supported by extensions if not in the core)
- looks good, have template mechanism, a few nice themes are available
- there are extensions to prevent spam, I don't know if an aksimet extension is available.
- it's mysql based
Optional:
- don't know what you mean by apis
- plugins and themes supported
Wet dream should be supported, at least in theory(on my old vanilla forum it worked in a manner I didn't want to, the code was html encoded, so html tags were not supported, including links, however i've seen them working on other forums), if not extensions could solve the problem.
Regarding the spam control and user comments posting you should use 2-3 plugins(Yes plugins are supported and are called Extensions).
