When Bob Dole waged his dismal campaign for President in 1996, I don't remember seeing stories about an "enthusiasm gap" nearly to the extent we see them today. What you did see were constant reminders of Clinton's yawning lead in the polls. Perhaps an enthusiasm gap was simply assumed. Or Republicans were plenty fired up about Bill Clinton, but were left without an effective electoral outlet.
Today, all we see are enthusiasm gap stories and their cousins -- the age meme, the Obama crowds meme, the McCain-doesn't-go-online meme. The media isn't attempting to bury McCain like it buried George H.W. Bush or Bob Dole, through bad polls. They're trying to bury him on the intangibles, chief among them the energy and grassroots enthusiasm gap between the two candidates.
I believe this shift from hard data to intangibles is a function of the Internet-driven base mobilization era in which we now live. The media has basically conceded that the election will be patterned after 2000 and 2004, in that it will be close. In close elections, polls can't tell you who'll win as reliably. So the emphasis is on derivative factors like the GOTV operation or crowd size or enthusiasm which will enable one candidate to outperform the polls. The Rove model, which 1) the media buys into, and 2) places a heavier emphasis on an energized base over tacking to the center, has perversely redounded to the benefit of Obama, who has the energized base this year.