My question is: With your prime character, did you miss anything that you regret now? Did you fail anything and were getting angry about yourself?
For example: My prime character is a Nord Warrior by the name of Magnus (funny by the way, I didn't knew beforehand that there would be the quest "The Eye of Magnus"

- I destroyed the Dark Brotherhood, instead of joining them. To be honest, Skyrim was my first TES game and when I came to the point, where I had to decide either to join or to destroy them, I simply thought that my character was a "good" character and it wouldn't suit him to join the DB. I wasn't aware of the fact, that joining them really has no or hardly an influence to anybody. I didn't knew that. Well, because of that I missed some unique followers (Cicero) and armors.
- I didn't know until a few days ago that you can collect 8 dragon priest mask to get a 9th mask. Even if the new mask isn't worth it, I would have liked it to complete this quest. Must be an awesome feeling to place all 8 masks and remember how you defeated every single one of those dragon priests. My problem is: I lost the Morokei mask and another one. I'm sure I defeated the dragon priests who were wearing them, but either I simply forgot to loot their bodies or I stored them anywhere and now I didn't know where anymore.
- At the daedra quest with Mehrunes Dagon I decided to reprieve Silus and therefore I were not able to get Mehrunes Razor. Again, this happened because I wasn't aware at that time that it really has no influence to anybody. This was probably my second or third Daedra quest and I simply thought, that I do not like to follow the orders of a Daedra because I'm a good guy. This is the only Daedric weapon I missed.

- I sent some interesting followers to the Blades (Mjoll, Kharjo and Aela) before I realized, that I do not like them very much. Unfortunately, I can't bring them to leave them (there really is nothing I can do, right?).
Aside from that, I'm really satisfied with my perfect character.

(note that I also have different characters and that I like to roleplay, this thread is just about your prime character if you have one)