A CMS without a learning curve I'd like to see. ;)
Gary is right though, you really have to evaluate what your site needs, what each candidate software provides and how well those two things mesh. Obviously we'd like you to use Tiki but I accept that it's not for everybody.
The general comment against Drupal is that many of it's features are not "core" features. For example there are at least half a dozen forum plugins/modules (whatever the correct terminology is) for Drupal AFAIK. If this "flag forum posts" only works with one forum, then you're tied to that particular forum plugin. In other words there might not be the apparent freedom of choice as first appears. It's all pros and cons though, careful and honest evaluation is needed based on your specific needs.