Nothing I can really note to be honest. Marshmallow is a pretty mature OS and I personally haven't found any fault in it. I reboot my device at least once a week. I had to reformat my SD card to have it 'seen' by the phone. Other than that, it's been a fantastic phone, hopefully others can chime in with any concerns/gripes.
As you probably know, updates for the "Active" series usually come a little slower than the regular S series, this is a AT&T unique device so updates always lag a few months behind the regular lineup. I'm OK with that given the battery life, which was the big selling point for me on this device.
Chris