Is it mainly used for our "main" page (with the introduction, no-articles, no-news), or our articles page? (a personal example would be www.skoogo.com , the front page has nothing but a nice clean intro. But inside it's a forum)
In blogs, I usually Stumble the first page and the posts. People usually won't look for interesting content when they land to a site, they just Stumble again right away. But if they land to an interesting post, they will more likely read the post and maybe look for more good content. Dunno with forums.. I wouldn't start looking for forum if I would land to your site.. Maybe you could try Stumbling some interesting posts/tutorials/information on your forum.
I don't really look at forums with SU, there are too many that are of bad quality. Blogs, if the post is well written and interesting I will not only read but might subscribe and comment. Make your site stand out. People see a lot and if yours is unique, they might take a look.
So is it possible to have a clean Web 2.0 front page with to-the-point introduction get popular hits with SU?
I would suggest you focus on posts more so than your frontpage. The average stumbler is looking for an eye catching headline and an intresting story, rather than a frontpage of various bits and pieces. In gerneral I have always had more success with SU this way.