I know Murlocs are kind of, well, mrghghghgh to play against in ranked, but no one can deny their effectiveness. I had very little trouble ranking up to Legendary from Rank 5 with nearly a 75% win rate on the way. Here is the deck I used. Mulliganing tips will be provided at the bottom.
2x Soulfire (0)
2x Power Overwhelming (1)
2x Voidwalker (1)
2x Flame Imp (1)
2x Young Priestess (1)
2x Murloc Raider (1)
2x Grimscale Oracle (1)
2x Murloc Tidecaller (1)
1x Leper Gnome (1)
2x Knife Juggler (2)
2x Murloc Tidehunter (2)
2x Bluegill Warrior (2)
2x Coldlight Seer(3)
2x Murloc Warleader (3)
1x Leeroy Jenkins (4)
1x Ol' Murk Eye (4)
1x Doomguard (5)
This is similar to a lot of murloc decklists, but certain cards many use (Ironbeak Owl, Mortal Coil) are not present. Instead we have a Doomguard, which won me many games. True, it anti-synergizes with Soulfire and to a lesser extent Leeroy, but if you can get an empty hand Doomguard off it is just amazing, and often discarding one card is no big deal. The idea was to get one more finisher-type card in hand to topdeck when your deck has stalled out.
Mulliganing: Generally mulligan away 3-drops unless your hand really synergizes with it. Always mulligan 4+ drops. Often keep a Soulfire but mulliganing it is okay if your hand has a hole in it. Always toss away Power Overwhelming.
If you go first, you want to drop a Flame Imp (ideal) or other 1-drop. Against hero power classes ideally you will play Voidwalker or Flame Imp, you don't really want him to take a coin-hero power on your 1-drop, although if it's Leper Gnome that's OK.
If you go 2nd, you generally want to coin into either Knife Juggler or two 1 drops.
Often after turn 4 (and almost always from turn 5 on) you will be tapping, so don't be afraid that your hand size seems to shrink dramatically on the first few turns. That is the idea of the deck. The only time you will need to be careful is against the opponent's AoE turn (so if you have a bunch of 1hp dudes on an empty board against a druid going into turn 4, you may wish to wait to see if he has Swipe before dumping your hand on the table, and lifetap instead. Same goes for control warlock). Basically, try not to play into cards you know he is running in his deck unless you need to.
Against another rush deck, especially Murlocs, board control is everything. Take favourable trades. Be willing to soulfire an early drop to take board control. Yes, it feels bizarre to soulfire his turn 1 Leper Gnome but if it combines to give you board control, I suggest you strongly consider it. Generally do soulfire away a turn 1 Flame Imp. Once you have board control your weenie murlocs will trade with his better murlocs and you will dominate the matchup. Your Doomguard will also dominate this matchup as your opponent will have nothing remotely that size (except Leeroy) to counter it.
Against a control or midrange deck we are trying to burn them down, but we still take obviously good trades. This is not a completely mindless "to the face!" hunter or warrior-style rush. If your opponent is able to wipe your board out and have creatures on the board, and have a lot of life remaining, you are going to lose, so you must take favourable trades when you can. The fact that our cards are easily buffed up by Power Overwhelming or the Murloc buffer cards, and the fact that we can Soulfire means that it is quite difficult for an opponent to stop us using only creatures, no matter what they are -- so take good trades! Even a Sunfuried Molten Giant is not nearly enough to stop a Murloc rush as you can just burn through it with your onboard position if he hasn't AoE'd you.
There is more skill in playing a Murloc deck than it seems (it really isn't totally mindless), but many of the plays are pretty scripted, and it's not particularly fun to play against. But, in the current meta it is super effective. And, it's kind of fun to play since you do have meaningful decisions to make on a lot of turns.
Also, it has the huge hidden upside that the games are extremely fast, so you might play 4 or 5 games in the same time a control druid might play a single game. Often, an opponent without AoE will concede on turn 3 or 4. This makes for fast and furius gameplay, which is good, and you can really take a philosophical view on the losses since, well, it's mrrgghghrhgrhgrgh! Good luck and have fun!