I was diagnosed with it 3 years ago. It was a misdiagnosis in my case, as I was recently told major depressive disorder is more accurate for me. Mood disorder NOS, though, as my former psychologist explained it is a diagnosis for those who don't fit the strict criteria for bipolar disorder or MDD. They fall somewhere in between. The reason I was misdiagnosed was because at the time I was participating in eating disorder behaviors. The effects of those behaviors were misconstrued as hypomania by the person who diagnosed me. But I didn't meet the criteria for bipolar type 1 or 2.

The diagnosis doesn't matter too much to me. I am glad I have a more accurate one now. MDD definitely feels more accurate for how I've felt most of my life. It is possible you're right about bipolar, but there may be a reason that diagnosis wasn't made. If certain criteria aren't met, a patient is generally diagnosed with an NOS disorder (such as OSFED formerly EDNOS for eating disorder patients who don't meet the criteria for one of the three more well known ones). NOS generally includes a lot of different symptoms. Unfortunately, this makes finding understanding of the diagnosis hard... that's the one thing I never liked about the NOS diagnosis.

Now, I may be wrong, but I believe mood disorder NOS could mean less severe depressive episodes, as well, such as the fact that I was thought to experience less severe hypomania than those dx with bipolar disorder.