Stop reading this -- go prepare for Yom Tov, print it out and read it later. I shouldn't be writing either, so this will be brief.
1. Chag haSukkos ta'aseh lecha -- as the Bnei Yissaschar explains, we have to let the chag penetrate and make us (lecha) into something different than we were beforehand.
2. Just two Shabbosos ago in Parshas VaYelech we read about the mitzvah of hakhel that was done every post-shemita year on the holiday of Sukkos. Why was the ceremony done at that time of year in particular? Ralbag explains (Toeles 13) that Sukkos being the culmination of all the three regalim is the pinnacle of spiritual heights that one can achieve during the year. Pesach and Shavuos and even Rosh haShana and Yom Kippur was just a warm up to get to this point.
He adds another reason: Sukkos is the harvest season and the abundance of newly harvested crops might entice one to feel overly self-satisfied and drift away from Hashem. Therefore, we need to inject ourselves with an added dose of yiras shamayim this time of year.
Even in years where we lack the added kedusha of a previously experienced shemita year and the added pomp of the king reading hakhel (Ralbag opines that the Kohen Gadol or a nasi could do the reading as well -- see the Minchas Chinuch who raises this as a safeik) the Ralbag's observations hold true.
3. Everybody is familiar with the idea of turning the esrog upside down before saying the bracha of al netilas lulav so that you get the bracha in right before doing the mitzvah, over l'asiyasan. But things are not so simple as they seem. No where does the gemara say to do this -- all we know from the gemara are two facts, one about the mitzvah of lulav, one about birchos hamitzvah in general: 1) Once you pick up the lulav, you have already done the mitvah of netilas lulav; 2) A birchas hamitzvah is usually recited before performing the mitzvah act. So now we are stuck. If you say the bracha before having anything in hand, that is considered too removed from being over l'asiyasan. If you already have the lulav in hand, the mitzvah is over and you can't say the bracha. How do you balance two mutually exclusive rules?
The Beis Yosef's solution (based on the Rosh) is that even though the gemara doesn't say it, we have to modify how we do the mitzvah of netilas lulav. We turn the esrog upside down (or don't pick it up or have in mind not to be yotzei yet) until after the bracha so it is over l'asiyasan. The focus is completely on working around fact #1 that we mentioned above.
It's the GR"A that reminds us of fact #2, and the simple reading of the gemara seems very much in line with his approach. The gemara (Pesachim 7) has a discussion of whether we should say birchas hamitzvah using the nusach of "al..." or "l...." At the end of the day some brachos are formulated one way, some the other, and the Rishonim all try to come up with rules and explain away the exceptions. At the heart of the debate is whether the nusach of "al..." refers to an action that is going to be done, i.e. the mitzvah about to be performed, or whether "al..." only refers to past events, and hence should not be used for a birchas hamitzvah which must be recited before doing the mitzvah. Asks the gemara:
מיתיבי העושה לולב לעצמו מברך שהחיינו וקימנו והגיענו לזמן הזה נטלו לצאת בו אומר אשר קדשנו במצותיו וצונו על נטילת לולב
Doesn't the fact that we say the bracha of "al netilas lulav" prove that a birchas hamitzvah can be formulated using the nusach of "al..." and not just "l..."?
Answers the gemara:
שאני התם דבעידנא דאגבהה נפק ביה
Lulav is different, as one is already yotzei once the lulav has been taken in hand.
In other words, you can't prove anything about other birchos hamitzvah from the bracha on lulav because lulav is a unique case -- it is one of the exceptions to the rule where Chazal allowed for the bracha to be said after the mitzvah has already been done! It's fact #2, the over l'asiyasan rule, that is modified (by Chazal) when it comes to lulav, not fact #1, how we do the mitzvah.
Tosfos on the spot (d'h b'idna) wants to have their cake and eat it and writes that the gemara does not negate the rule of over l'asiyasan because you still have na'anu'im in hallel to do. The implicit assumption is that na'anu'im is part and parcel of the mitzvah of netilas lulav and not a separate kiyum, but that a different discussion that I don't have time for now.