As much as I love the High Line (some photos from my excursion last year), I share David's concerns that the development of it will not turn out so well:
The problem is that linear parks don't really ever function as parks, a place to hang around and enjoy nature, they are often built (like the highline) in a place that does not lend itself to mature planet growth and the spaces themselves are not 'static' - in short they become expensive, fancy, shrub lined, bike lanes.
The double whammy for the Highline project is that it is a raised linear park, with all of the problems that separating pedestrian flow from the ground produced in large urbanism projects in the 50's and 60's.
The really unfortunate thing about it is that the High Line is really cool and I would love to see it developed into something great. Walking along it, you get a unique view of Manhattan, both literally and figuratively. And from below, it just looks cool, especially when you catch people up there looking down on you. I think of the High Line as a bit like TiVo was a few years ago...difficult to explain to people who had never seen it, hard to understand why you'd ever need such a thing if you'd never used it, but once you'd used it for more than 10 minutes, it's hard to imagine how you ever did without it. And so it is with the High Line; it's hard to understand the appeal unless you've been up there. But as David notes, the linearity and elevation may make it difficult for many to find their way up there and discover that for themselves.