The violence at the Spurs game in April 1978 had a big effect on attendances IMO. Perimeter fencing went up during the 78/79 season but the violence at the West Ham game put lots off. That was certainly given as the main reason why the first game in the top flight v Arsenal was attended by...
Just for info, at The Goldstone, we had 21 attendances over 30,000
Since we've been at The Amex, we've had 29, although one of those (v Wolves - 30,003 in May 2013 was a bit spurious)