The following can be read on:  http://en.wikipedia.org/wiki/DotNetNuke, how about this criticsm? Is it still valid for this projekt?

Criticisms

There is in general a lack of support for cross-platform, free software runtimes such as Mono.

DotNetNuke also suffers from a number of limitations inherent to the ASP.NET framework itself. Support for browsers other than Microsoft Internet Explorer is poor, for example, and some users have reported difficulty in developing standards-compliant and tableless skins. The default skins and the core framework use a table-based layout and the DocType declaration is hard coded into the core framework, necessitating at least minor tweaks to the core if standards-compliant XHTML output is required.[1]

However, as these issues have been largely addressed in version 2 of the .net framework, these problems are present to a lesser extent in the latest version of DotNetNuke.

ASP.NET support just does not work like it should be on platforms other then IIS/.NET so cross platform support is not yet an issue. depending how you look at it, tying deeper to IIS could even be an advantage. If you really need Linux, go for PHPnuke

Dotnetnuke is HTML 4 compliant, I am shure they will switch to XHTML compliance when the need to support .NET 1.1 is dropped

yes the core modules use Tables, nothing wrong with that.

poor browser support is just a lie, there was a small FireFox issue wich was related to the .NET framework being older then FireFox. It was fixed ages ago