Battery life. This is one of the biggest selling points to me. I'm replacing a Fitbit Charge 2 that would last around a week or less. Previous to that I had a Mi Band 1S and one of the best things about it was how long it could go between charges. I'm really looking forward to getting my Mi Band 4. 

According to this XDA article and this article, the Mi Band 4 has a 20 day battery life. That's about 3 times as much battery life as my Fitbit had. 

I ordered a Mi Band 4 not long ago and it should get here next week. I'm looking forward to it.