I think the answer's in the question. You typed "ghost" in quotations, and I think that was wise because Spike wasn't the type of ghost our gang was used to dealing with. For one, he was the ghost of a vampire, and for another he was chained to earth through unusual means (the amulet, Lindsey, etc). Dennis couldn't make a visual manifestation of himself either, but Spike could. Other ghosts in the Buffyverse had different qualities than Spike as well, such as the possessing ghosts in Buffy's "I Only Have Eyes For You." They could only repeat their last life moments while possessing living people.

I don't know if Joss ever established a set "rules" for ghosts on either Buffy or Angel. But even if he did, I'm inclined to believe that Spike (special circumstances as they were) would have broken most of those rules anyway.