I think you're going to have to give your deck list to get a catered response. Some mid-range don't run Flametongues, for example, and some do - so having one already in hand during the mulligan can affect what else you would throw back to search for, depending on what class you are up against. Hopefully someone else can add to this to be more informative.Against warlocks, you really want a lightning storm or some other AoE/multi-target cards. To be more specific, you want cards that trade 2 for 1 or better. Warlocks draw power means we have to be card efficient versus the aggro warlocks, and handlock is slow enough that you can get away with mulliganing like they are aggro. You can still win versus a handlock if you mulligan like they are aggro, but you will rarely win against aggro if you mulligan like they were control. Also keep earth shocks, because that is good versus either warlock type.Against hunters, you do NOT want an AoE or multi-target card in hand, but multi-minion like FS is still good here. If you run a Doomhammer (or any weapon), keep it - it's often vital to racing a hunter down, and can help you determine if a trap is an Explosive or Freezing/Misdirection.Against priest, if I happen to have a 4 attack minion in my opening hand, depending on casting cost, I might just keep it depending on what else I need to mulligan as well - KNOWING you will have a minion that Priest can't remove is a lot better than hoping you draw one, and Priests play slow, so you can get away with this usually. Feral Spirits are good early, as Shadow Madness runs one wolf into another wolf and neither die. Lower strength minions are iffy due to the Clerics - 1 health minions are a no-no.Against rogue, you want taunts. Whether it's aggro/tempo rogue or miracle rogue, you need taunts. The difference is in when you play them - if it's miracle rogue, don't play taunts before Gadgetzan is on the board unless you can put out 3+ based on your hand. Against shaman, you want lightning storm. Shaman games are often decided by who gets the first and better value lightning storm, especially if Feral Spirits are on the board, in my experience - the game snowballs out of control from there against the loser of the dice roll.Against warrior, this one is a little tougher to mulligan for. It's rare to run into any non-control warriors, although they are out there, so you generally want to mulligan like it's a control warrior. Regardless, the easiest way to beat a control warrior is totems - Shamans can generally remove one minion per turn even when they are big ones, which is what this warrior arche-type essentially plays like. So when you get to the point you have all four totems and all they can do is kill your stoneclaw totem each turn, you'll usually win eventually just by default, heh. Still some luck involved, of course. Against mage, they don't really have any standard copy-cat builds going around right now. So just mulligan for a strong opening, and maybe a sacrificial lamb to eat a Frostbolt. Avoid 1 health minions, obviously. Against druid, I tend to keep a lava burst if I have one, or even a hex. Almost every druid deck I know of runs legendaries the hex will eventually be useful against, but both deal with the turn 1 or 2 Yetis while minimizing your damage taken if that happens, and token druid isn't as popular now as ancient watcher druids are. Better to hex an early yeti than be down 12-16 health by the time you can kill it. Against paladin, this one is almost as annoying as warlock since there are two types I run into - aggro pally, where I need a storm/card efficiency, and control pally, which is a lot more forgiving. Aggro pally runs out of gas relatively quickly, so good removal efficiency will see you through against them most times. However, even aggro pallies tend to start slow for some reason, summoning a dude on turn 2 instead of a minion. This makes Feral Spirits difficult for them to recover from if they don't play a 3/2 on turn 2.
Then there are the specific examples that require multiple things to be going on. For example, if you are playing against a Paladin and going second, if a Mana Tide Totem shows up in starting hand, I would never throw that away in that match-up. If a Paladin doesn't play a 3 attack 2 drop (which is pretty often), you are almost guaranteed two card draws, minimum, from a Mana Tide against them - Paladin CLASS can't deal with that on the Paladin's turn 3, because Hammer of Wrath is 4 mana, as is Truesilver Champion. If you then manage to get taunt up on turn 3 (especially Feral Spirits), that opening alone will usually win the entire game since it's rare for Pallys to even run Hammer of Wrath these days.
But you see how specific that match-up is for what to keep? Each class match-up has examples just like this, so it's kind of hard to list them ALL out, especially if you aren't even using some of the cards we could list. :)
Some general tips though:
Edited by Ranilin on 4/15/2014 1:35 AM PDT