Conservancies are nice, but they don't really have the funding or the mandate to make the kind of drastic design changes the Greenway requires in order to become a vibrant public space. The name says it all: we'll get a group that lovingly (though god knows how) "conserves" a space we know is a failure. What we need is some kind of Jane Jacobs/Olmstead crossbreed superhero to radically reshape it.
And no, I don't like Post Office Square. The highly-lauded high plantings make it feel cut off from the rest of the city in a way that does a complete disservice to the empty corporate boulevards that flank its sides, and it serves the seasonal midday lunch crowd and practically no one else (contrast this to its New York equivalent, Bryant Park, which is practically overbooked with programming that lasts through the winter). The consensus that this is a great public space is ridiculous. It's a high-strung mutual fund manager's fifteen-minute recreational safety valve.
Oh, and as for the rest of you just now making points like "give the Greenway time" and "maybe nothing could have been built on the Greenway" - try checking the last 38 pages of this thread to see where and how these ideas have been addressed - over and over and over and over again. By the way, Columbus Center is being built over the Mass Pike as we speak, and this ill-conceived space is going to require a lot more than matured foliage to thrive.