You can't just rush through the stages and find the green stars at the end. The opening camera shots rarely show the locations of green stars, so you have to search for them. That falls under exploration, albeit not extensive exploration, because Galaxy 2's levels are rather linear. Hence why I said "some need for exploration".
They should've given us new and more challenging ground to traverse in each galaxy. The stars would be hidden through out the new grounds and you would receive no help in finding them. If you happened to pass one you would be able to go back, most of the time. This would've made the green stars more enjoyable. Also it got rather annoying to see the comet warning every time you got to the star select screen. `