Just for anyone who may not know how exactly playtesting for >G works. Heck, I am not even 100% sure what happens before the first batch are posted for us to use. However, from what I have gathered, the >G crew actually playtest quite a bit before it arrives for us. So most mechanics and even cards a pretty set in stone by the time they arrive to the Forum Playtesters. Currently the only deck that has gone through a complete redesign is Setback (sorry, I hope this isn't rubbing it in anyone's face, I try my best not to come off that way), most other decks only have some cards altered (such as increasing/decreasing damage/health), some sub text are moved from more powerful cards to the weaker ones, and the occasional card replacement. Don't mistake this as me saying that Playtesters have no say, a lot of the changes that a made are direct request of playtesters.
Argent Adept was really insane when he was first introduced, Syncopated Onslaught and Counterpoint Bulwark worked on all hero targets rather than just 2. Plus the Accompany on Alacritous Subdominant didn't destroy itself. Early on Rythyms were only Performs while Melodies had both Perform and Accompany. He also had a target in his deck, that had HP equal to the Instruments plus 2 or 3. What that target did was allow Argent to activate a accompany immediately after that accompany was activated. He pratically made all the heroes invincible, while making them a wrecking crew.
NightMist really didn't change much, most of her changes were card name changes. The biggest change I can think of is the Amulet however (so fortunate for that one) which would let her draw a card at the start of her turn, but at the end of her turn she would have to discard one. Really not that great, especially compared to the current. Also I Heedless Lash (at the time called Energy Blast) discarded the revealed card instead of putting it into play or hand.
Unity's biggest change was making it easier to put bots into play. Before she would have to destroy 2 equipments for her base power, discard 2 for Modular Workbench, and Construction Pylon only put 1 into play. I also don't think she had a way to heal bots in the beginning. Most other changes were HP totals of the bots.
Akash'Bhuta and Ennead I believe went mostly unchanged. At least I can't think of anything...
GloomWeaver was severely weakened. However I think there has been a thread about this at one point.
Apostate's changes consisted mostly of HP totals. However Corrupted Effegy had another name, and worked like Undaunted. Runes of Malediction only gave a DR1, but also gave it to Demons. While the Orb only redirect damage from Apostate.
I can't recall any drastic changes from the environments however.
Edit: I shall toss this in, I was a bad playtester for Shattered Timelines. So I can't say anything I remember from those.