As the title states, treasures at the moment are extremely powerful and way way too random in being acquired. Just using a recent example, I played two priest runs in a row, in which one of the choices for my first treasure was Grommash's gauntlets... That's right, -1 cost for equipping weapons, on a priest. This basically removes one treasure as no one will ever pick that, and if your other 2 treasures are horrible, your run is basically over before it even begins with how prevalent aggro decks like DH/Hunter are. This game mode has so much potential, but when stuff like this happens, it makes it kind of obvious that no one really put and deep thought behind it, it makes one think that the game mode was just added as a further gold sink/way to get some $$ out of people that want something new out of Hearthstone.
My first instinct was "Eh, that's not a problem, the fun is seeing how well you can do with a high/low roll", but that's because I've been playing exclusively Casual Duels (spent my two free entry tickets on Heroic just to cash them in and that's about it). The treasures as are I think work really well when you're basically just screwing around and trying wacky builds. I wouldn't want to change them for Casual, since some of the funnest moments I've had were trying to pivot into a Dragon deck after I got a Dragon passive, or stuffing my deck with every Deathrattle I could after picking up a Mummy Magic, stuff like that. In Heroic I can see how it could be way more frustrating. At the very least it should ensure that you're only getting treasures your starting deck could reasonably have been built to support, like only offering Gromm's Gauntlets to Weapon classes, and only offering Tribal support cards to classes that have support for those Tribes (I doubt a Rogue would ever be taking Dragon support, for instance). At that point you're at least giving players the option to build around the possibility you'll be offered the treasure (Yes, Dragon Priest may be suboptimal, but what if you roll Dragon synergy? Do you want to put some Dragon support in your starting deck just in case) rather than including treasures that are effectively a dead option.
Also, Robes of Shrinking needs to go. It's a worse Scrying Orb and it never doesn't feel awful.