The Don Valley was an awful ground and is certainly up there for me as well as one of the worst. Another one that's sprung to mind is Brighton's Withdean Stadium. Temporary stands, a racetrack around it like the Don Valley and completely devoid of any atmosphere and so far removed from the action it's easy to lose interest.
One of the worst bowls for me was Darlington's. A 25,000 seater stadium yet they only got crowds of a few thousand. In terms of % full compared to capacity it must have been one of if not the lowest in the football league. No atmosphere whatsoever.
Having said that great memories of us going in the Munto season when beat Darlington 5-0 and secured the league two title.