As a longtime player of the midbrackets, I feel each one currently has something worthwhile to offer.
39s are in a neat place right now. The second row of talents provides good options for many classes. A variety of gear exists to customize according to playstyle, and while 49s and above offer a few more gear options, there isn't any must-have gear at 39 to be effective on the field (unlike 49 and 59). Getting 95% BiS stat-wise at 39 is fairly straightforward, and while obtaining that last 5% can be a royal pain in the ass (definitely harder than 49s or 59s because certain 39 scaled BoEs are so rare), it hearkens to the glory days of gearing up for swag appeal.
Discs and fury warriors are top dogs, but not as extreme as 49s and 59s. Like any bracket, healers are really strong thanks to their crazy off-damage. The stat gap between twinks and levelers at 39 is smaller compared to brackets below and above, so blowouts happen a little less often.
As others said, the main appeal for 39s resides in its pace. The damage-to-health ratio sits lower than the other brackets. When I want to play a hero with broad shoulders, I do 49s or 59s. When I want a different kind of intensity where I need to read the field more, 39s call to me.