Here is some insight on why I build Ana this way and how I use her to be effective on the battlefield.
[*] DBM - This talent requires Ana to be a bit more aggressive in early fights to get her stacks. While enemies are less dangerous at lower levels, keep your head on a swivel and get your stacks smart. Don't over extend for stacks. Remember, the goal is high healing. No one is getting healed if Ana is dead.
[*] I like Overdose at LVL 4 because its a talent that synergies well with your LVL 1. Land the dart and you're instantly 3 stacks in on your goal for that target. Two more AA's on target and you're a step closer to Quest completion.
[*] This is a crossroads choice for me. I like Temporary Blindness because we are removing steps to proc talents in a sense (or rather, we are not adding steps). Our goal is to stack doses. We are "sleeping" to both neutralize targets while simultaneously stacking toward our quest to get our Burst Healing Potential. Blinding targets for 2 seconds who are coming out of sleep allows your team to get closer to securing those desired kills. As an alternative, Mind-Numbing Agent is super strong and synergies incredibly powerfully against mages. An Instant 30%+ reduced spell power on a target coming out of sleep is insanely potent. So I like to go Blind versus Raynor or Butcher and such, where I'll pick MNA against a Jaina, Gul'Dan or Kael'thas.
[*] Nano Boost - Always. I had a bit of fun with Eye of Horus at first after coming over from Overwatch and trying something new on my favorite hero, but nothing compares to +200 Mana, +30 Spell Power and +150 Cooldown recharge time on your DMG focused ally. When used on a Genji, Tracer, Maiev, Illidan, and even a Jaina or Diablo, the effects are devastating to the enemy.
[*] Another crossroads choice. Pay close attention to what is being used effectively against your team and how things are being applied. Are they being rooted or slowed or are they being stunned or both? How is the enemy applying these effects if they are using both against your team? Is it a stun to a slow or a root to a stun. Your awareness and play-style could be the difference in saving your team. Do you prevent the root that leads to the stun or do you answer the inevitable stun that is going to occur after a root or slow and then guarantee some amount of armor to your teammate when you apply the heal? You should be very close to your quest at this point if not completed.
[*] Everything at 16 is honestly really good. Really, there is no wrong choice here. There is, however, what I feel is a "better choice". Because we are in the mode of stacking doses and are built out to have doses applied in our LVL 4, we want to go with Concentrated doses. Lets look at why;
For us to receive the same value from Sharpshooter, we have to not miss the previous Healing Dart for the added 5%. So to get 10%, we have to have hit our last dart, then hit this dart. But If we have AA'd on one single enemy hero, we are guaranteed to be at that same value with this outgoing Healing Dart. If we have full stacks on an enemy and then sleep a different enemy, we are exceeding the max healing output of Sharpshooter by 30%. So you're auto attacking and at 5 stacks we match the max of Sharpshooter without having to concern ourselves with past accuracy (having had to chain 10 Healing Darts without missing).
Earlier in this tier, even back at 2 AA's on an enemy we are better off on our next dart than having hit our last 2 healing darts on the next outgoing dart. Work smarter not harder.
Contact Healing is amazing however again, we are getting to that +55% healing by having to do more work after we land the nade because beyond the 166 healing on contact, we are having to try to land another shot to effect our friendly Hero. The purpose of having our quests was to get to instant available heals. As in, "I see a friendly in trouble, here is health with a bonus". We are not looking for, "I see a friendly in trouble, let me use a cooldown so I can use another cooldown to heal with bonus". Smarter not Harder. Still, 30% per ally and enemy hit means if you drop a nade in the middle of a team fight where allies and enemies are clustered, the healing you can then apply with your completed quest is about unmatched outside of a few ultimates. 30% More per Ally and Enemy. Add to that the initial 25% on target for 4 seconds and you are quickly fire off around 935 in healing rapidly with your completed quest when you grenade 4 heroes.
[*] Somnolent Doses - because along with the synergy from every other aforementioned talent choice, what could be better than putting a target you just slept (basically stunned), right back to sleep after they are slept, woke and blinded or weakened and in danger. By having those active 3 doses you are healing your heal and close to AA sleeping a target at this point when you have landed a sleep dart already. Makes escaping death very hard against Ana.

Ana is about being aware. Any support should be aware; its what the position is all about. Ana should not hesitate to call shots and move her team around. She should not move onto the desired call out prior to her team rotating. If she leaves too early her team could die. If she leaves to late she could be caught by chasers. Ana has a very limited mobility and its always a good idea to mount up after a call to move. Even before the team disengages. You can always dismount to save a trapped or dying ally, but not being ready to roll and being stuck in chasing enemy CC could lead to Ana's death or worse, that of teammates who have turned to try to bail Ana out. Be ready to move.
Learn to kite and position. This is so important with all supports. Positioning is huge. This includes probable enemy attack directions from which foes would arrive while your not only team fighting, but taking camps and objectives. Do not be he first one in the line of approach. That's a tanks job really.
And as always, knowing your opponents will guide you to adaptation. Rather than concerning yourself as much with the score screen, instead follow the talent screen and see where your allies and opponent are guiding their builds.
Have fun with Ana and feel free to tweet your thoughts at me; SLOWIIIII