anjohl: None, most modern top tier games are fine as is. This is self evident, since if the amatwur modders work improved the original product, then they would be professional designers, not amateurs.
Most of the really good modders are professionals. Anyone else remember Alien_Slof from way back when in Morrowind? And Bethesda farms ideas back in from community modders all the time. Where do you think horses came from in Oblivion? Or Hardcore mode in FNV? Both were ripped straight from incredibly popular mods made for their predecessors, and I've heard rumors that some very skilled modders have been approached by Bethesda.
You're entitled to your own opinion, of course, but I think the Gothic 3 community would disagree with you as well, considering the fan patch made that game viable (and the company later abused that support and killed their own franchise, but that's another story).