Save your trade contracts until you have all of the non-hidden mounts.
There's only one real choice that would affect partying, as there are achievements tied to it so people may want specific ones even outside of wanting their story to go a certain way. It's a fairly early choice, however, so once you get past that there is nothing as far as choices that would make partying a bad idea as far as I know.
You will need 6 mastery points (tier 3 of the first mount) to get the second mount, which you will need to get to progress the story after a certain point. That is the only part of the story that is mastery locked, as once you have unlocked the second mount you have high enough mastery by that point to also unlock the third one. There are more than enough accessible masteries between story and the first map to get the first mount to tier 3. :)
Take some CC skills if you've got them in story instances and stability is nice for some of the mobs as well. Condi cleanse is nice too.
Have fun! I hope none of that was too spoilery.