Maiev Shadowsong has singlehandedly been the final straw that lost me 4 games out of 8 playing embiggen druid into DH. Whenever I've finally locked down the board and started to be able to play up multiple taunts when I'm low on HP they always have a maiev for my 8/12 or 9/9 taunts and swing face after killing the other taunt for lethal.
Definitely one of the stronger – if not strongest – legendaries this patch. Coming in close are all DH ones, Evocation, both paladin legendaries, and then a few spotted around the place like Magtheridon that can be really potent with Flamestrike, Hellfire or Flamereaper.
Silence is a lot different, make it goes Dormant remove 1 space from enemy board and remove a minion for 2 turns while Silence remove only the minion effect. es. if your opponent have a 30/30 and you silence it, the next turn you will lose otherwise if you make it goes dormant the next turn you will not lose.
without talking about using [card]Maiev Shadowsong[card] for active your dormant minion effect another time.
(sorry, if you see some english mistakes, I made my best)
Maiev is great because it can be used not just for finnishing enemy by having effect similiar to silence, but it can also help you gain tempo. You effectively disable opponents minion for two turns and also gain 4/3 on board. That all just for 4 mana. In next two turns you might by such a powerfull tempo swing even kill your opponent, so the minion you "dormated" wont even return from dormat. Because of this, you can even read mayevs effect as "Destroy enemy minion".
Basically whenever you play maiev, you are doing the right play.
A lot of people are playing Maiev Shadowsong wrong. They use it as a hard removal when it fact it is the softest removal in the game. It should only be used for getting rid of a taunt to cause massive face damage and ONLY defensively if you know you can take out the big threat you're making dormant when it reawakens along with anything else the enemy summons.
Al'ar is insanely good is rez priest. It is extremely hard to deal with for slowly decks and If you can hit it with reborn or the deathrattle summon 2 copies, it easily overwhelms the enemy.