system
Given the following post, respond as if you're commenting on Reddit:
human
[LoTR] What makes Balrogs different from other Maiar?

What makes a Maia a Balrog? Does the balrogs have any specific traits that differentiate them from other Maiar?

If it is just that they used to serve Melkor, then why is Sauron not a Balrog?
gpt
Not all  Maiar who served Morgoth are Balrogs. The Balrogs are Maiar of Fire who took physical form.  Sauron, is not the final part. He is not a Maia of Fire and he is not a Balrog.  

Gandalf is an example of a Maia of fire who is not a Balrog.