Gentrification vs Prosperity

What, exactly, is the difference between gentrification and prosperity?
I mean if the neighborhood you live in is prosperous, the people around you are doing well and thriving – isn’t that a good thing?
Perhaps the problem is that “market value” is too often being driven by forces that have nothing to do with the individual prosperity of the residents… forces such as “city planners”, zoning boards, and corporate developers.

Let’s take Union Square as an example like this article does http://www.bostonglobe.com/opinion/2014/03/04/mcmorrow/HzAPfrtdTAsL9BrxINAcsL/story.html
What is gentrification anyway? Seems to me that “gentrification” is one of those weird subjective vocabulary words. Perhaps it means something like this: “There’s something that I want/need that is priced outside of my range to purchase it.” Perhaps sometimes it means old delicatessens and dive bars turning over to multi-national corporate banks
The problem in my mind is not that new housing and commercial rental units are being planned and built – it’s that our “common wealth”, namely our land, is being handed over to corporate developers, rather than ownership staying in the hands of the people who need it and belong to it.
The people who live and work in Union Square, the people who make it what it is today, are the ones who should be directly benefiting from development opportunities.But instead we are all like refugees worried about getting pushed out. Or maybe that’s just me?