The other day when I was helping Sceadeau understand what I think about when I draft I kept using the term 'value'. It's something I've picked up from watching streamers of the game and it's a pretty developed concept I think, but as someone new to the game Sceadeau didn't really know what I meant. I mean, obviously you want to take cards with higher value than other cards... That's what the word means... But how do you weigh the value of different cards in a draft? He asked and I answered as best I could. I've been thinking about it more lately, and here's where I've arrived.
The first thing to understand as someone who comes from a game like Magic is creatures in Hearthstone are designed to die. Creatures in Magic heal to full at the end of each turn which means it's entirely reasonable to expect a good durable creature to live several turns and eat several cards from the opponent in the process. Creatures in Hearthstone rarely heal, and never automatically. The permanent damage on them means that you play a creature, do some damage with it, and then watch it die.
The end consequence of this is the default assumption for a creature is you're going to trade it for a creature that costs the same amount from your opponent. So the basic idea of value that I'm using is that it's what you expect to have left over after trading with an average card of the same cost from your opponent.
For example, how good is spider tank? It's a 3/4 for 3. If you trade it for a 3/3 taunt guy you get to keep a 3/1 and they keep nothing. That's pretty good! Harvest golem? Spider tank remains as a 3/2, golem leaves behind a 2/1. Tank has the edge here, but if nothing else goes on the 3/2 just trades for the 2/1. Dalaran mage? Spider tank straight up stays as a 3/2 against nothing. Harvest golem stays as a 2/1 and then another 2/1. Even the taunt guy sticks around as a 3/1 taunt. Dalaran mage is terrible under this comparison.
Always comparing with something of the same cost isn't exactly fair. Fighting up or down one in cost is a pretty standard situation too, and it does help to think about those situations. There are also hero powers that can be relevant. Sticking around as a 3/1 is a lot better against a warlock than it is against a mage, for example!
You shouldn't go too crazy with comparing up or down though. Obviously an archmage trades very well with a wisp, but it also costs 7 more mana!
What sort of things are typically found on cards I'd consider to provide high value? Big stats compared to other creatures is a good start. 4/5 yetis for 4 are surprisingly good even when they have no ability. It's just super hard for any creature of equal or lower value to kill a yeti in one round, and similarly hard for one of those creatures to survive a round with the yeti.
The stat mix is also really relevant. You want fairly balanced stats, with toughness actually being better than power most of the time. Lost tallstrider is a 5/4 for 4 and it's actually a lot worse than the 4/5 yeti. The problem is the 5th power tends to be wasted against creatures around the same cost. They don't have 5 toughness, so in a trade the extra power isn't used. Having only 4 toughness increases the number of creatures that will kill it is one shot. Especially against a mage which can tag in an extra damage for 2 mana if they need to do it. But while 4/5 is better than 5/4, 1/8 is not better than 4/5.
The 5th toughness is especially important because of the existence of flamestrike. Having creatures you can play before the mage's turn 7 that live through flamestrike is clutch.
Creatures that are 'sticky' can also provide good value for their cost. Stickiness refers to a creature that generates an extra body in some way after it 'dies'. This can come in the form of divine shield, or a deathrattle effect which generates an extra creature, or even a battlecry which generates an extra creature. The battlecry ones tend to be weaker since mass removal can hit both creatures at the same time, but it is still a potential source of value.
Or maybe it's a creature that generates card advantage in some other way. Maybe it has a good sized body for the cost and lets you draw a card when it comes into play or dies. Or maybe it does some bonus damage at some point. Demolisher is a 1/4 for 3 which is a terrible body, but if it stays in play it gets to do 2 extra damage each turn. It's generally pretty terrible value, but I had one game today where I had enough removal via weapons that I was able to keep a demolisher alive for many turns and it just kept picking off small paladin creatures. It was pretty good value, that one time.
It feels like creatures often pay some amount of power or toughness to get an ability on them, and then that ability just doesn't do much. Spellpower, windfury, and taunt are some of the big ones. When they work out they do great things, but if you spend 6 mana for a 4/5 windfury and the enemy just trades for it right away you never get to attack twice and you ended up spending 2 more mana than a yeti for just a yeti. That sucks. The 2/2 spellpower guy for 2 can be good when you follow him up with a swipe or something, but if instead he just gets eaten by a 2/3 then he was low value for you.