Developers are being shown that their apps — and their months or years of hard work, and in many cases, their entire businesses — can be yanked by Apple's whim at any time for reasons that they couldn't have anticipated or avoided. This invokes fear and anger from many, and I think most of the “30% is too much” arguing is a misdirected side effect of this frustration not at the number itself, but at the seemingly arbitrary and greedy nature of the rule.
Well said. For the record, I'm almost certain Apple will tinker with the subscription policy before the June 30th deadline to officially exempt services1 from the rule. That aside, it's still a troubling precedent which leaves the door open to further abrupt rule changes — what if Apple decides that they're entitled to 30% of all ad revenue, or simply outlaw third-party ad providers altogether?2 Plenty of people seem to think they'd never go that far, but nobody really expected them to demand 30% of all subscription revenue, along with mandated price-matching.
Jobs said as much in a characteristically thrifty email, but nothing's been made official. Also, if they do make certain exceptions, how are they going to delineate content subscriptions (National Geographic) from service subscriptions (Dropbox)? If they never intended to include paid services in the new rules, why not say so from the start?
Realistically speaking, they'd have to ban other ad providers: it's the only enforceable solution.