The Impossible Achievement, never finishing Les Miserable II and the Peat Bog Quest, and never being able to wake up on time to get the Down Under The Wire badge (usually, feats end at 5 am my time.) also, never getting all my Completist badges and finding all my DNA spots.
But honestly, all of those are technicalities. For someone who only played for two months, I managed to get a LOT out of those two months.
I can't say I have any regrets (because I loved playing the game and enjoyed the way I played) but there are a few things I wish I did:
I wish I had gotten rid of my stash of teal-white triangle keys when they were actually worth something instead of holding on too long and the key market crashing
I wish I actually launched my Glitchen's Court idea (a pseudo small claims court for minor squabbles). I drafted court rules and later created a courtroom in my tower. But, again, my timing stunk - between the unlaunching and trying to deal with the community guidelines about not calling people out, I never got it off the ground.
I wish I played each race game and visited each region. But, at the end, I focused on the regions that interested me most (I LOVED the underwater regions and the ice regions)
I found out about the Juju quest after the game closes.
I did not quite get to level 50.
I forgot to visit Drifa.
I never released a Pocket Salmon.
Regrets? None.
I wish I had found Glitch sooner, and understood sooner what a gem it was. I started in October, and then didn't play again for almost a month!
I wish I had taken more snaps, and sooner, and realized you could snap an entire street. I wish I had taken a snap when I left the No-No Powder on the subway under its poster.
I wish I had the faintest idea what my birth street was; it never occurred to me to note which one I first wandered out onto. I wandered mapless for many days because I didn't realize I had one!
I wish I had played long enough to know my birthday.
I wish I had played music blocks for myself, not just for crabs.
I wish I had noticed global chat earlier, and used it before the final two days.
I wish I had done more exploring, and sooner; I could have won completist in so many more regions if I had realized that was a thing.
I wish I had explored Hell at least one of the times I was there.
I wish I had understood sooner why my butler kept telling me about rook attacks. As it was, I didn't finish learning Martial Imagination until they were no longer occurring.
I wish I had noted the name and friended the person I encountered somewhere in Alakol and chatted with the night before the end.
I am grateful that I actually managed to level up from 3 all the way to 31 between Nov 16 and Dec 9.