A commenter is dubious (see this post from a few days ago) that we could really have a "park" under a retrofitted viaduct. I tend to agree. A "park" in the sylvan sense might be tough. But we can certainly do better than what we have now.
Here's an example of what Paris has done: Beyond The Big Dig —Viaduc des Arts. Lots of photos. (Robert Campbell in the Boston Globe)
More info here at Viaduc des Arts : 45 craftsmen and creators in Paris.
Why not create cheap commercial spaces under a retrofitted viaduct? If you really want to generate excitement along the waterfront, a mile of cheap work/studio space for artists and craftspeople under a repaired viaduct will go a lot father than a lawn. (I've never favored the idea that "What downtown Seattle really needs..." is more "open space," which is, btw, a term I have never liked.)
Obviously, this vision is a very different one that those set forth by either the Mayor or the People's Waterfront Coalition. But I think that it is worth considering. My observation (my own preference aside, to the extent one can ever be ruthlessly objective) is that we, the Seattle body politic, will conclude that
1. the Mayor's vision is not within our budget;
2. it's pointless to build a brand-new viaduct if we can do without one for the projected seven year construction period;
3. the People's Waterfront vision is too risky, too chancy, based on too many assumptions about how things will work just right.
I have to say that while I admire the pluck and boldness of the visions offered by the Mayor and the People's Waterfront, I just don't think they are timely.
The vision set forth here — let's call it the "repair with work-space underneath" model — is affordable, practical and urbane.