A loaded word. In the general context of the term, it's an unhealthy progression that takes an imbalance of concentrated poverty and results in an imbalance of economic and social exclusion that too often also falls upon racial / ethnic lines, resulting in lack of diversity (socio-economic).
In terms of a socio-economic ecosystem, New York is a perfect example of pricing out the lower middle class and now middle class in many neighborhoods, creating a situation where a vast majority of the workforce can not afford to live in the areas that workforce serves. It also has a blanching effect (pun somewhat intended) in terms of mix of stores and restaurants, which become repeats of themselves from gentrified hood to gentrified hood.
Of course, another result is that the poorer populations as they can no longer afford must find areas to relocate, so rather than having a balance of economic classes spread throughout a region, their will be new pockets of poverty. As the center city cores are being most gentrified, this has a double impact as poor populations are pushed further from the work centers and further from the more dense neighborhoods best served by transit, forcing higher transportation costs and more auto dependency which both splinters communities and also adds addition financial burden.
So, long story short, in the traditional sense, gentrification takes the imbalance of poor and pushes it to an imbalance of wealthy, and also a lack of socio-economic diversity. That's not healthy for a community nor a city. It's best to find balance, to find ways to have mixed-income communities that are far healthier, long term, from an ecosystem and sustainability perspective. Those cities that can find the right balance are best positioned for long term fiscal health and stability and social well being.