I've seen some sukkahs (especially ones where they've had big "make sukkah decorations" kids' projects) where there are tons and tons and tons of decorations hanging down from the schach. At what point would that invalidate the sukkah? Are there other decoration issues that are problematic?
As long as they are within 4 tefachim of the sechach, they are batel to the sechach- even a lot of decorations. (S.A. O.C. 627:4)