One of the pet peeves I have with Arena is that you can have more than 4 of the same card. If it's from two different sets. You still are not allowed to play more than 4, but you can have 8 Llanowar Elves since they appear in multiple sets. The difference is entirely cosmetic, but you don't get wildcards instead of useless cards.
Jesus Christ plays esper
I'd like to do a Jund Dino set, but I don't wanna spend all those wildcards.
Sultai cavaliers ... so much fun to play
She doesn't have First Strike, and dies to most 2mcc kill spells. The deck is easy to spot. Got Sirens? Got Miscreants? Got Shitbirds? You got a Sephara deck. It gets overpoweringly devastated by Ritual of Soot, Cry of Carn, the Boros 3 damage AOE spell, etc.
It's fun to play though when it goes off.