The liberal wing of the court plus Justice Kennedy formed the majority in this case. Justice Steven's opinion relies heavily on precedent and provides little actual argument. It says that based on previous Court rulings we should take a broad view of the term "Public Use" in the Fifth Amendment, one that allows us to include this type of private redevelopment. The Court should defer to the CT legislatures view of what is good for the public. As is typical of many of the opinions from left, this strikes me as nothing more than declaring Constitutional anything that the judges feel is good public policy.
Private property is a fundamental right that is guaranteed by our Constitution. It is no less important than freedom of speech, or freedom from self incrimination, or any of the other rights enshrined in the Bill of Rights. If the Government has the right to take away our property at will, simply because they feel someone else can make better use of it then we don't have any property rights. Justice Thomas' dissent makes this powerfully clear and actually provides an argument and not just a list of other cases.
Thomas also points out that this ruling from the liberal wing produces a result that is generally un-liberal. Eminent domain of this will usually be employed against the poor and possibly minorities. Thus this ruling allows the government to take away property rights of those who are generally disadvantaged in order to aid the wealthy and powerful (the developers). Such a wonderful liberal result! Though I suppose one that could be justified by some on the left who take a highly paternalistic view ("we're going to help these people by getting them out of blighted communities, whether they want to or not"). I suppose one argue that this makes the ruling one of principle and not of policy preference, but the other facts in the case argue against that interpretation.
Near the end of the majority opinion Justice Stevens says that while he doesn't believe the federal constitution prohibits this kind of taking, various state laws and constitutions do so people who would oppose this should turn to their States for a remedy. Ah, federalism! But if this is the case then why don't we return to the original (pre-Fourteenth Amendment) interpretation and say that the Bill of Rights doesn't apply to the states? Then we can all appeal to the states without any of this pesky federal involvement, at least when it's not Congress doing the taking.
Update: The following comment on a thread at the Volokh Conspiracy discussing the case expresses my feeling perfectly:
Can someone explain to this non-lawyer how it can be that such a very basic issue about the very old concept of eminent domain can be new before the Supreme Court?
Some outrages are so outrageous, it can take a couple of centuries for someone to work up the nerve to perpetrate them, and then actually try to defend them in court.