Lots of people seem to have problems with X spells (and/or basic algebra). MaRo recently said:
kalebmulder asked: Is X costing too complex for common in NWO? or NNWO ? :P
Having an X in a mana cost is a red flag under New World Order meaning we don’t do it at common unless there is a strong reason and we count it as eating up some of the complexity at common.
I personally never put X spells at common in my sets as experience has taught me that a subset of players have a real hard time with X.
What if Wizards used reminder text on such commons to help teach players how they work (and how to do simple algebra)?
"(When casting this spell, first choose a value for X, then replace all the X's on this card with that value)"
I am sure there are plenty of cards that could go into a Core Set at common with X on them to help illustrate how they work.