It sounds ridiculous, but tablet-dedicated forum is unreadable on my tablet (N800, OS2008). The biggest problem I have is those small buttons (arrows after dates, "Quote", "Edit" etc ) are enlarged and cover almost full screen . I have screenshots but don't know yet how to insert them here.
I did some changes to the about:config to set minimum fonts but I am pretty sure I had same experience before.
Another minor problems I would point to are very slow rendering and very massive graphic header, which cover almost all first page. In other words, page is too heavy for a tablets. Don't we have mobile light version?