I'm been tinkering around with Serra Avatar ever since it came out way back when, in Urza's Saga. The best you could ever get it to be was a "win more" deck with Armadillo Cloaks and Spirit Links and things like that, to double your life every time you attack.
When I saw Mayael's Aria for the first time over on MTGSalvation's spoiler list, I knew the time had come! (Alright, I'm getting a little excited, but it's been so long looking for the perfect use for the Avatar.)
Think about this: You only need 9 life with both Serra Avatar and Mayael's Aria out during your upkeep to win the game.
The key is the wording on the Aria. It has 3 separate effects and they all occur in order, one after the other. If you have both cards out and 9 life during your upkeep, you will satisfy the first effect by having a creature with 5+ power. All your creatures will get a +1/+1 counter -- including the Avatar. Next, you'll now have a creature with 10 power, giving you an additional 10 life. Finally, you'll now have a creature with 20 power. You win the game.
[Decklist] Avatar's Aria
3 Serra Avatar
1 Ajani Goldman
3 Mayael's Aria
2 Loxodon Hierarch
1 Firemane Angel
4 Birds of Paradise
4 Swords to Plowshares
4 Careful Study
1 Fact or Fiction
2 Deep Analysis
1 Crop Rotation
3 Hunting Grounds
1 Zuran Orb
2 Armadillo Cloak
4 Mox Diamond
1 Sol Ring
4 Maze of Ith
1 Diamond Valley
4 Riftstone Portal
4 Windswept Heath
4 Tropical Island
First thing is the inclusion of 1 Ajani Goldman. It's just that good. I swapped out the original 4th Avatar for the plainswalker. The life gain (obviously) works with the deck, but the +1/+1 vigilance also works with the Hierarchs and Firemane Angel. That extra +1/+1 will put them ever the edge if you have an Aria in play.
Another thing you'll notice when you start playing with it, is that while the Avatar / Aria combo is the center piece of the deck, you can easily win the old fashioned way.
If you've been reading my blog (and if you continue to read it) you'll start noticing that there are some cards that I use in a lot of my decks. I have my favorites. One of those cards is Careful Study. Card drawing, discard, and graveyard possibilities all in one. And whenever I include it, the second card I always consider if it's right for the deck is Riftstone Portal. Once you got that 2 card combo going, all of the lands that usually don't produce mana are fair game. Throw in a set of now unrestricted Mox Diamonds and add a Crop Rotation for style points, and all of a sudden everything starts coming together.
I've always liked Maze of Ith. I always try to fit it into decks, but it usually never fits. It does in this deck. If dealing 20 damage to your opponent is becoming a problem, no worries. Just turtle up, and wait until you can draw into the Avatar + Aria. All gravy.
Diamond Valley is the same. I'm tempted to add a second copy to the deck... but with only 11 creatures in the deck it's hard to justify it. Still, Hierarchs now give you 8 life, and you can insta-sac Avatars for double life if they're threatened.
Swords to Plowshares and Zuran Orb have their tricks, but I don't think I have to spell them out.
There are some casual decks that I like to play more than others, and this is my favorite type of deck. Crazy combo? Check. Lots of cards that no one really plays? Check. Synergy among wacky cards? Check.
All-in-all this is one of the best decks that I've ever made. It won't win you every game, but when you do, it's a lot of fun.