There's a few things I regret, and things I would do again but wish I could change the outcome, and things I just wish I could do.
Regrets: After playing with a couple friends for 3 years and agreeing to all do CoP together, I went on and did most of it without them. I would have liked to do it with them, but they just are not on often enough. I regret promising to do it with them to begin with. (Even after promising to go back and do it again for them, they are still not too happy with me. But 3 years! I can't wait forever.)
I regret buying a drk his haub and some ugly scythe that he just HAD to have. 2 weeks later, he quit. His best friend (69 pld) took the gear and paid me back for them, then a while later he quit so the gear ended up with my husband when the best friend was made into the UltraMegaMule. (er, that made no sense, so here: the pld was my husband's brother, the drk was his best friend. The pld is now one of my husband's mules.) So no harm done, but still. The drk might have had a better time in the game if he had earned his own way.
I regret dropping all my beastmen seals when I was a level 5 nublet because I didn't know what they were for and couldn't sell or eat them.
Would so again but wish for a different outcome: After playing on an endgame ls for several months, we switched ls's. The one we were on was just not right for us. I would still wish to leave, but I wish it wouldn't have hurt some of the other members' feelings.
Wish I could do: I wish I could tell Aldo that his sister and her kid are chilling in Selbina with amnesia. I wish I could tell that one mithra kid in Windurst how to get to the SOB meeting. I wish I could change the color of my tail. I wish I could send my moogle out of my mog house to get a pic of my house with no moogle. I wish I could make people understand that when my /sea comment says: "CoP Mission. Party, No Thanks. I'm sorry, I'm busy now." That I don't want to go manaburn on blm. I wish I could talk my two best buds in game into getting along with each other. And now anyone bored enough to read down this far wishes I would not write big huge blocky walls of text.