Anyone know how the power level stat work?
It's just basically a flat modifier to everything you do. It takes your average hero power to calculate, which uses both hero power from your level(10*level) and from your equipped gear(totalgear/5). There's also "tresholds", your damage doesn't increase linearily but in small increments, for example there's a video of a guy showing normal melee attacks doing 275dmg on body shots at 194 hero power, and then at 195 it bumps to 300per hit, and stays there all the way to 240+(and like 160 is also 275 hits).
It not only affects damage obviously, but also cleave power(amount of enemies you cleave through with horizontal swings) and stagger power.
Basically, the more the better, but if you have a tradeoff between a piece with better mods but lower hero power, you have to go to the dummies and hit them a few times to figure out if you're losing a treshold or not. If you aren't, then it seems pretty irrelevant so you might as well use the lower one with better mods, but if you do it'll generally mean you want to keep the higher power one. Also different weapon types seem to have different tresholds.
Note that power is also gated by difficulty, boxes from higher difficulty have higher caps on it regardless of your level, so you get more powerful gear at higher difficulties(so you can do even higher difficulties, makes sense really).
Finally on the subject of loot and power levels, it seems it's better to keep the commendation boxes(the ones you get for gaining levels) until 25/30. They supposedly scale all the way regardless, so opening them all at 30 would give you a tons of lvl 30 gear, while opening them as you go would give you pretty meh gear which you probably won't need. It isn't super confirmed yet though and might have changed on the current patch. That said you seem to get quite a lot of gear regardless so no big deal.