The first challenge that new teams often face is a lack of alignment. Usually happens when goals aren’t clear and common, and when there is ambiguity of roles and responsibilities. As a result team members quickly get siloed in their own agendas and egos instead of being focused on overall team results.
The second challenge new teams often face is when team members are overly polite with each other. A client of mine recently described it as "being around the in-laws after just having been married. You're on your best behavior because you're not in a place of being totally comfortable." In the workplace that results in a lack of open dialogue, avoidance of challenging others about their opinions, or even harder, failing to hold peers accountable.
The third challenge new team members face is a lack of clarity of team norms. Norms will happen on a team regardless of whether they are intentional or not. And when they're not intentional, bad habits tend to creep in. For example, I have a client whose meetings start 7 minutes late. For individual team members the key is to arrive late enough not to have to wait too long for the others, but not so late as to be the last one in the conference room. That's a norm. And, it's not particularly productive.
Overcome these three team challenges, and you'll accelerate your team's effectiveness.