Hearthpwn

Wow.

What? Are you serious?

Just no.

I even got Ragnaros, Lightlord.

You can even get Tirion Fordring and the mighty Millhouse.

And still there is literally no reason to change the rarity of the card. Be happy that it is common, so that also budget players have access to minions with higher rarity.

Commons like A Light in the Darkness actually serve a really nice purpose in Hearthstone right now, in that they give a cheap and accessible option for free-to-players to play with legendaries and epics that might otherwise be inaccessible, as well as enhancing the flavor and variety of cards available in Arena. It might produce a mine of salt, but having a card that can add Rags and Deathwings to decks that otherwise couldn't access them helps to spice up the game and present more interesting decisions throughout. Really that's a big part of what makes discover such a great mechanic.

You effectively rolled a die, got lucky.

Can now play a 10 mana Ragnaros.

And something about card rarity, what was that?

I'm assuming you have started similar threads for Flamestrike and all the other 50 cards that have questionable rarity before you started to look at this card.