Let me explain.

First of all, you flame buffet has descent range and let's you fight without needing to get anywhere close to that pesky Fenix on the enemy team.

Secondly the talents!
Alexstrasza's healing is focused on getting her dragon form, she is very powerful and can change the tide of battle in seconds used correctly.

Your first talent let's you reduce the cooldown of dragon, so you can use it more.

The second talent slows the enemy for a lot initially, so they can't get to you nor can they walk away as easily.

Tertiary, you heal for 10% for every enemy hit, this equals, more sustain, more Q-healing and much more. You level 13 is interchangeable, I just think Pacify is very useful.

Then 16 and 20, if you have a dragon form that lasts longer and applies the cooldown reduction of the level 1. You get an Alexstrasza that basically always has close to full HP, always has dragon ready to go and that slows everyone. And remember, Alexstasza's strongest point, is her Dragon.

I think that it is because I am low rank, but I always see Alexstasza's with the 75% hp build...
But this is so bad, if you build this, you are really not helping your team!

Let me put up a scenario, you build the 75% HP build. You are 70% HP at the moment, you don't have any talents. You are a talent-less Alexstrazsa who can't sustain herself, can't heal her team enough and gets shutdowned by taking any substantial amount of damage. The level 4 speed boost is close to useless, and the level 16 armour is almost unnoticeable.

I hope someone takes this guide seriously as I would love to see more Alexstasza's helping the team out.