If I don't have enough mana for one or more cards in my hand, I'd like them to appear "grayed out", so I know they aren't useable.
Great suggestion, ekted. We can see how this would be helpful for making quick judgements on available tactics and perhaps even speed up game flow as a result.
There are some interface questions to consider. At the end of your turn, all of your left over mana is drained into your hand as new cards (or vaporizes if your hand is full). Effectively, this would mean all cards that cost more than 0 would be grayed out during your opponents' turns which may or may not be a visual concern for some players- particularly because the 0 cost cards would still be lit, implying you could cast during your opponent's turn. We're curious what you and other players think in how this could be handled- perhaps during your opponents turn all cards gray out, maybe the spell bar dims or the cards get smaller, etc. Another possibility might be, instead of dimming the cards, to indicate in the card rollover popup that you do not have enough mana by showing the cost in red.
I would simply go for graying out all spells during your opponents' turns. I agree that this would simplify and quicken play.