The first factor of elves being my favorite bloodline may partly be my heckin good luck when I opened my packs, I got a gold summoner(always a very nice thing to have) and Sacrosanct Shanker whose a great front liner to have as they have cripple.
When considering that there is a healer in the starter cards AND quite a few people have the epic card Aracne Weaver which does corrupted healing. With corrupted healing the creatures are healed but at the cost of a reduced maximum health, when this is coupled with cripple, things get interesting.
The other thing I really enjoy about the elves is the execute ability given by the archon Kaylia Silverleaf. This ability means that if a unit is almost dead(1 or 2 health left), or is just a low health unit, they get a second attack after their normal attack. This means cards like silverleaf warlock or a level 2 arcane skinwalker could take out a 4 health unit in one go if they don’t have a defence against magic. This ability gets even more useful when the elves get to go first thanks to say, having a lvl 2 silverleaf warlock that has boosted everyone’s speed. I have one thanks to my awesome friend 
There is of course downsides to this ability, for one, if an elf only has one attack power, and someone has shield or void armour, it's not helpful, and if a unit has any of the abilities that retaliate, that ability will be triggered twice. For this reason I avoid playing them when someone has tofu, or in rule sets with multiple retaliation effects.
The little league rule set also makes elves less then ideal as the only two cards playable in modern little league rule set is the archon and Sacrosanct Shanker. Which I've done with decent results but prefer the 3 mana dual splinter summoners for little league.
Playing elves in low mana rulesets can also be tricky as they tend to cost 5-6 mana, so if you've only got 13-15 mana, that limits choies a lot and could mean only have 1-3 units out when going for lower cost cards could be better.
And now I'd like to show you a fight in which the elven execute carried the fight.
Here's our starting line up.
I wouldn't personally put a shock tropper in this high of a mana game. Buuut with the weaver I could see them thinking they could compenstate for it. However I don't think it works well with equal opportunity rule set. Though with the taunt that's less of an issue.
So round one done, everyones still standing and I was starting to get a wee bit worried.
The armour is gone from the tank, and their max hp is now 7, that said my own tuant unit is almost dead. >< And she's expensive costing 11 mana.
A few turns later, things start getting more tense.
My tuant unit, Silverleaf Mage has gone down, but we also have the first execute, from the skinwalker! Which downs the demon.
So thanks to the speed boost from the warlock and magic going first in a tie, the rest of their team goes down, plus another execute, this time from Aranduriel.
Then with the Zapped, the warlock takes out the weaver.
Alas the shock tropper takes out my front liner.
But then he gets executed.
And the same happens to the last member of the opposing team.
Alright, so I've got comment rewarder enabled. I think it's an awesome thingo and a great way to encourage good conversation.
So what do you think is the best bloodline or best feature of the new cards?