I think there are many great filters, but I think one of those filters is fighting over limited resources and wars. Perhaps limited to humans/earth, but I doubt it. Nukes, dropping rocks from orbit, and theoretical (but possible) weapons like black hole bombs are all going to tempt irrational beings to take someone's stuff.
We have to be extremely careful that we don't accidentally trigger a weapon that is going to kill or dramatically cripple our civilization before we become a truly interstellar species. There is so much to learn out there, while so many people are currently focused on the wrong things such as minor conflicts or what children aren't allowed to learn.