You can definitely play with just the base came and Road to Legend. In fact, RtL doesn't make use of a lot of the extra rules additions the expansions included, so if you're finding yourself wanting that campaign experience, go ahead and get it.

thalcos is right. However, Well of Darkness does add a lot nice stuff to RtL (mainly treachery, but also interesting monsters, skills, and items) that it makes it a much better game. As for Altar of Dispair, many of the rules added in that expansion are not used in RtL (but you still use all the other stuff).

While the rules are not really used, I have to admit, if I didn't have all the expansions, it would feel like an incomplete game. If anything, just for the extra monsters to use. Setting up the boards and being able to choose between different monsters is more fulfilling, so I would highly recommend picking up the expansions.

-shnar