I only struggled with a couple concepts when I joined. The first was creatures not blocking, seemed kinda strange. I guess it does speed games up though.
2nd was immortals not being targeted by spells that affected 'all'. Was a bit backwards to my way of thinking.
Oh, and still don't understand why Sundials stop immortals.
So, I guess 3 things
![Smiley :)](https://elementscommunity.org/forum/Smileys/solosmileys/smiley.gif)
I also cut my teeth on D&D (well, AD&D) and moved on to Magic (just online, not the physical cards), then here.