Personally, and I've mentioned this many times previously, I do think there should be some sort of personal report card for each (or at least certain select) dungeons and trials. Not shared with the group, not posted anywhere publicly, just one that pops up upon completion of the Duty and gives you basic letter grades and performance pointers. It's not going to help anyone learn who doesn't want to learn, but for those who do want to learn it may help push them in the right direction or lead them to ask about how to improve. Of course, this would be aided tremendously if the game itself were better at teaching players how to play the game without forcing them to go to a third party resource.

As for commendations, it would be nice if there were some sort of use for them but I don't really think there needs to be one as that could just lead to more issues anyway. Besides, a lot of players don't even commend based on performance anyway so it would be near impossible to nail down as an actual performance reward. Heck, I know that I will often tend to Commend DPS if I notice they're playing well as I know that Tank/Healer are the default for most people regardless, or will just Commend someone who happens to chat a bit and be friendly/helpful. I'll also tend to Commend a player who says they're new to the Duty just as pat on the back for getting through it. I wouldn't say performance is the last thing I consider but it's rarely the first unless someone really went out of their way and did something special.