Something I just noticed : 1430 url index on google.com
vs 14 100 url indexed on google.nl
Could be because your site is hosted in Netherland, could be worth to look for an uk or us ip if you're truly targeting an english audience.
Also, the page loading time and the huge amount of useless (at least seowise) content on the forum index are not very good for your indexing and SERPs. I see that the guest message is hidden to bots, but that's not enough (and could actually be taken as cloaking by google), the shoutbox should be hidden too, and better, placed with the welcome message on another page.