Heroes is darned good at it, making you yearn for the action, and only barely following through -- even in the finales. But when this same trend hits the big screen as well, the irks increase. This experience has been paid for; the expectations are higher. And in the case of Harry Potter and the Half-Blood Prince, it comes after a number of hours of dealing a packed, antsy theater of prepubescents. <spoiler alert>
Eric's already mentioned one of the big changes between the book and the film -- just how Harry watches the Snape/Dumbledore confrontation unfold. But what about the fight downstairs? Hogwarts wasn't picking their noses when this all went down. And most importantly -- Rowling had finally let Professor McGonagall get her fight on as she Ginny, Tonks, Lupin, Neville, etc battled the Death Eaters. Surely that's better than Bellatrix tromping around? Surely room could've been made for it. Take out a snog or two. There still would've been plenty to giggle about. The gem of the final books is seeing everyone get their due, whether that be revenge or the chance to finally see them fight and come into their own.
There's no reason for The Heroes Effect. If action is hinted, if cool things are happening -- show them! If the book's got a great scene of fighting that has McGonagall as a robe-clad Ripley, show it! If you can't ... why bother?