I am exploring a reason to upgrade from Views 2x to 3x.
What major benefits Views 3x provides? Are there any major API changes?
There are hundreds of improvements. There are also many API changes, but they would only affect custom code using Views hooks, not the views themselves, which are converted upon update.
Some major features:
Most importantly, it provides a much smoother upgrade to Views 7.x-3.x.
See the CHANGELOG.txt for a complete list of changes.