Is it immoral for me to not take a Lulav on Sukkos?
I would not call that immoral at all. It is sad that someone would deliverately lose out on the opportunity to fulfill a mitzvah, but that is a bechira that affects only yourself, and you are not punished for it, merely lose out on the s’char mitzvah (unless I am grossly mistaken, in which case someone please enlighten me).
Morality, IMO, relates primarily to mitzvos bein adam l’chaveiro, not necessarily to mitzvos that are solely bein adam l’Makom (as your Lulav example was). If you choose to not use a lulav on Succos, you are simply not getting the mitzvah of Lulav. It is not immoral I guess, but it certainly is the failure to do a mitzvah, which is not a GOOD thing.
Most immoral acts seem to connect in some way whether directly or remotely by extrapolation, to one of the three yehareig v’al yaavor mitzvos, if we think about it. Even if one rightfully points out that “just” stealing or cheating on taxes is not one of the three cardinal sins, the way I see it the worship of money (both literal and figurative Mammon), is a form of A”Z, which leads to many ethical fiscal aveiros,and can subsequently lead to the other two cardinal sins for real. ALL cheating stems from the feeling that one can and should take whatever he so desires (ultimately that potentially leads to arayos). I don’t think that is such a stretch, if we are intellectually honest about it. Of course, this is all my opinion and no one has to agree with it.
If you want a concise definition of morality from me, you’re outta luck, Sam. I do not get to define morality, only Hashem does. Which is what I have been saying all along…