The Beit Din of Rabban Gamliel, and the parameters of shemitah, and how they are derived. What can be done before Rosh Hashanah, including when it comes to saplings, and how might that differ from mature trees? What does it mean that Rabban Gamliel's court nullified these prohibitions? What mechanism of halakhic reasoning even allows for that?! Clearly, not everyone agrees. R. Yishmael and R. Akiva simply have different methods of interpretation, which practical applications. Plus, why we don't irrigate a field with rain water, because, really, what's the great exertion in that? If the water comes from a running stream that is constantly fed from a pool - that seems to be good enough for irrigating in Chol HaMoed. But of course, there's machloket over this irrigation question as well... Where the difference in rainfall in Babylonia and Israel may make a very practical difference.