Ok talking about old stuff, but a while back sites like Heartharena did studies about win rates and card quality of the deck in question...

Now while I don't have a quote, AFAIK it was fairly clear, especially in average and non-infinite players, that there was a direct correlation between average score of your deck and how well you ended up doing.

The change can be described as this...

They got rid of the top 3 non-Kara class cards from Rogue and Mage. Left Paladin unchanged. Removed the 6 worst Arena class commons from the rest.

This will have a huge impact in the odds of building a good deck. The basic idea being, that you can make a 12 win Priest or Warrior deck, it was just highly unlikely.

Also AFAIK, class card offering rates are relatively fix. Meaning that much like the Discover mechanic, the game decides if you are going to be offered a class card first, with increased chances of doing so and then pulls from the pool of available cards of that type. This means that removing most of the crap cards will fairly dramatically increase the chance of making a solid arena deck.

All of that said... Arena is still a zero-sum game. If you were a 4 win player... This won't magically make your average better unless you were actively trying to avoid playing Rogue or Mage before.

Personally I think they might be taking it a little bit too far because people might not understand the impact of removing pretty much all of a classes bad class commons. However... the change will be fun to test.