“What you’re going to see is a campaign that has clear direction, but not a Simpson-Bowles or Ryan-budget level of detail. It’s not only politically unwise to do that, but it’s not how the voters engage in a presidential campaign.”
He also said
“The nature of running a presidential campaign is that you’re communicating direction to the American people. Campaigns that are about specifics, particularly in today’s environment, get tripped up.”
It's completely legit if they want to run a campaign on "direction". But they are making specific promises, like 12 million jobs created by the end of his first term, that the rich will pay the same share of taxes and the middle class will get a cut (despite analysis (.pdf) to the contrary), or balancing the budget by the end of a second term while not touching Medicare or Social Security and expanding military spending. Specific promises have to be backed up by specific proposals, or else they can't be taken seriously. Obama has to put out specific plans, by virtue of having to govern, so why shouldn't Romney?
If Romney's plan is to slide into office without actually proposing anything, why would he choose Ryan, a man who causes a very specific plan to pop into voters' minds, as his running mate? In the absence of specific Romney proposals, voters are left with specific (and unpopular) Ryan ideas.