Two main factors to keep in mind here:
* It's a new property. Remember, the cast will not be featured in A:AOU. So, while it will undoubtedly see an "Avengers boost", it might not be as pronounced as in the case of IM3 or T:TDW or CA:TWS.
* It simply doesn't have as much cultural cache as some of Marvel's other properties. While one could cite the previous point and argue that GOTG was in the same boat, the GA almost uniformly found that property "cool" as soon as the first trailer hit. They're not quite there with Ant-Man yet.
A second trailer will be extremely important to ratchet up the GA's currently somewhat lukewarm excitement level up a couple of notches. Also extremely important, more so than any other Marvel franchise might be the WOM and critical reception.
I think Marvel will, however, ultimately deliver a good movie and will create more buzz going forward. I like the concept of superhero legacies, etc., I like the cast, I trust Marvel's creative vision and I trust the Marvel marketing machine.
I think Marvel's calculus would be as follows:
$350 mill: We messed up. Let's learn from this and move forward.
$400 mill: We didn't necessarily "mess up", but this is a slight disappointment none the less.
$450 mill: Ok. No celebrations, but we can live with this.
$500 mill: Good. This is a positive result. We can possibly start talking sequel now.
$550 mill: Good ol' Marvel magic.
$600+ mill: Muuuaaahaha. Four words: Squirrel Girl... sex comedy. We're green lighting this just to see how far we can take this.