One of things I like about the Green Box of Games is that the icons aren't strictly defined, and lend themselves to being different things. I can't really think of a great term for it, but I'm thinking something like themed abstract.
The icons definitely match each other in style - but what are they exactly? Is the #6 icon a Tree, an Arrow, a Light pole? Is the #3 icon the end of three Logs, three Tires? a Storm system seen from space? 3 Targets? It all depends on the game.
I made up a game for the GBoG called "Gear Head" in which all the icons are parts of a car. I am not sure I did the greatest job in theming the parts (the #6 was a "Shifter" - which kind of works, but there's probably a better use for it).
Another advantage is it's based on 6 instead of 4. This allows for a wider range of things to be done.
Keep in mind, I'm not bashing the Piecepack, and I know there are other variants - so you could combine the standard piecepack with the Suits piecepack to get 6 suits (or the Seasons one). And there's a Hexpack.
I also know the Piecepack has been around much longer and has many more games than the GBoG - and more community support. However, I don't think it's a competition.
Having the GBoG opens up new ways of playing. It's just another tool to add to the mix. Not only do I own the Very First Edition - and have Kickstarted the Second to None Edition. It also inspired me to buy a huge box of colored cubes to add more possibilities - and a bunch of discs (specifically Bingo counters) to add another dimension.
I think it could be used in tandem with other game systems like the Piecepack or the Rainbow Deck or Looney Pyramids. That's what I plan to do anyway. :)