My forum has been inactive for the past few months due to lack of time to set it up running over a new forum software, but in my previous experience running a busy free hosting forum, I believe it's all about the kind of sponsors you are choosing to advertise, banner size and ads distribution.
Having AdSense and/or a contextual advertising slots scattered in between (or inside) posts is usually non-intrusive and good way to monetize, as it was AdSense 728x90 leaderboard in the header, not actually due to the new textual adverts format but still good if you make that unit images-only.
Other than this, sponsors whose banners, products and services are in accordance with the forum context don't scare members away and can be useful for them. In example, if you have a forum with a section devote to weddings, but the rest of the content is about many other topics, you cannot make available the associated banner across all the sections of your forum, but will still work inside that specific section.
It's about common sense, your own visual appreciation to avoid advertising over-cluttering and a careful planning of sponsored placements.