Are there any things you did when you started playing the game that now you regret? Let's make a list so we can all learn from each other's mistakes and help newer players as well.

I'd start with 3 of my own:

Very early on I built common and uncommon champions like Sister Militant, Yeoman, Death Hound, Saurus, and Outlaw Monk all the way to 50 to serve different roles in a young account. That took lots of resources that, I'd better spent elsewhere. They ended up being 5-50 chicken. The only uncommons I don't regret building are Armiger and Shieldguard https://1921681254.mx/  

I fed Woodpainted. When she sucked before her buff. Learned my lesson. Never feed an epic.

I fused Rhazid as soon as I got to him. The problem with this is that, unlike the other permanent fusions, you can fuse him, and all the epics you need for him once. Once you have done that the opportunity of fusing him is gone. Why that is an issue? For a couple of reasons: In the champion chase events for fusions and fragment champions, I like to fuse. I fuse 5 relic keepers from scratch every time, and having the opportunity to fuse others would have helped. Also, I would not mind having a couple of copies of Tortureheim for FW and secret rooms.

Those are 3 of my regrets. What are yours?