X is four.

This is also much worse than the previous "discard X cards" cards. This is horrible card disadvantage and I very much doubt it'd be worth it in most cases. It's also not really blue at all.

Oddly, I think the card suffers from being able to target creatures, because then players will get Alex's negative impression. They'll compare it to a card like Char and go "Ick! Discard 4 cards to deal four damage to a creature? No thank you!" The fact that the card is Arcane doesn't help this mentality either, since Arcane spells are spells you cast casually in order to get random boosts off of it... but this isn't a casual spell, and more often than not, players will cast this with X as 0 in order to splice a spell off it, and that will feel horrible.

What I suspect Alex isn't quite noticing, however, is that this card is a very powerful Kaervek's Spite. It's a finisher. The best use of the card is to dome another player for damage equal to the number of cards in your hand. Discarding the cards, when used in this mode, is irrelevant, since your opponent is dead. I presume that's why blue is in the cost of this card also--it's because it's a red card that's best employed in a deck that plays with Kami of the Crescent Moon and Spellbook.

If anything, the card is overpowered, but people won't see it until they play with it. And I think they're more likely to play with it if it doesn't include the option to target creatures (and at that rate, becomes a sorcery.)

Oh, and here's an alternative wording for the card that won't cause as many negative gut reactions:

Mind's Arrow
1UR
Sorcery
If target opponent's life total is less than or equal to the number of cards in your hand, that player loses the game.

Technically, that's more powerful in a number of different ways (especially in multiplayer.) It's also more of a blue spell since it directly expresses that knowledge is power (not direct damage.)

Even as a "very powerful finisher", this is much much worse than Firestorm, and usually much worse than Nahiri's Wrath. It's comparable to Conflagrate without the front side (or where you're only allowed to choose X=0 for the front side).

Firestorm is too old to make any proper modern comparisons, but I also remember a lot of players complaining about that card's power level, especially back in the day when Necropotence was legal. Nahiri's Wrath doesn't hit players.

I forgot about Conflagrate, though, which you're right about. You can pretty much do better than Mind's Arrow with Conflagorate by adding to your mana pool and dealing 0 damage on the first go round.

With the current wording, you can infinitely discard 0 cards for 1 damage each time.

EDIT: I'm sorry. The mis-wording made me reach this as a permanent. I didn't look at the card type.