Unanswered questions:
The main issue with the series that bothered me was all the unanswered questions. Admittedly I never read the manga, so they may have answered some there, and some may have been answered in the manga after the anime ended, so I wouldn't know those either. Here are the ones I can think of right now. I may add more later.
1. How exactly is it that Yoruichi can become a cat? Is it a family trait? Is it just a skill she learned personally? Is it something else entirely?
2. What is, and what are the abilities of Yoruichi's zanpakuto? Unless I am mistaken, you NEVER see her actually use it. She just uses shunko instead.
3. Ichigo's dad appears to be a captain. When, and of what squad? I heard that may have been answered in later manga's, but not sure.
4. When they did the backstory of the Vizard, it was mentioned that there was a new Kenpachi. That was too long ago to be the current one we know, and from what little of his description we did get, he doesn't seem to be the one the current one defeated, so who was he?
5. What exactly is Komamura? Anthropomorphic dog/wolf just isn't a complete enough answer to me because there are no other anthropomorphic animals ever shown in the series (outside of the zanpakuto's manifested forms).
6. If Orihime had never gotten those hairpins, how would her powers have manifested?
7. Can Chad get a similar kind of armor over his entire body, or just his arms?
8. Does Tatsuki have any actual power beyond what was given to her by Urahara for the whole Karakura Raizer thing?
9. Who/What exactly is the Soul King? He/It is referenced several times, but almost nothing is known about him/it beyond the fact that he/it is integral to the existence of the Soul Society.
10. Does Nel ever regain the ability to retain her adult size in a more permanent way, and does she ever return? I have heard rumors that she (as an adult) and Grimmjow do return at some point, but nothing about how or why.
11. When Byakuya uses his Senkei Senbonzakura Kageyoshi on Ichigo he says that Ichigo is only the 2nd to ever see it. Who was the first?
12. In season one, when Rukia first sees a Menos Grande she says they can only be take out by the Royal Special Task Force, and no single soul reaper can take one out. By the end of the series though she and the rest of Ichigo's crew are taking out groups of them on their own. And by that time, single soul reapers are taking out Arrancar and even Espada, which are basically "Super Menos." How does that work, considering that there is no way their power levels could have increased THAT much?
Hopefully somebody can answer at least some of these for me.