The general preconditions would be the same as those for watching any live Taiko drumming performance - An enjoyment of the music, and a facination for the skill / performance of the player. The arcade version leaves less room for "show ponying" compared to actual Taiko drumming, but then the songs being performed are more topical / popular.

Just looking at the picture the player seems to have all the signs of a skilled player - the taping and the fact that he's using his own sticks (No strings / padding, so they aren't the ones that come with the machine).

Also, as someone who until just recently put in an hour or so a day on the drumming machines (I know, but they get addictive... and you can kid yourself that they're good exercise), if you've been playing yourself and see someone doing better - especially much better than you can - then there's an added level of appreciation of the skill involved. And of course when YOU nail a song it's nice if the people watching look impressed too (I've only been clapped once, and that was by drunks, so it probably doesn't count).

Also, in a busier arcade you usually only get three songs and then let someone else have a turn. After a while a certain... comradery forms even with complete strangers.

Still, that's bigger than any crowd I've ever seen, even in Akihabara, was it some sort of event?