Okay look: People who play "Wall of Thorns" strategies, bring plenty of support spells to ensure it works, he will have Dispels, Disenchants, Steal Enchantment, Steal Equipment, Detonate....and more than one wall of Thorns.
It's not like you're going to counter a well constructed Wall of Thorns book with just a few spells, you really have to commit a fair percentage of your spell book to counter it. And since it deals such a massive amount of damage so quickly, I mean is there anything that deals the amount of damage per mana spent than pushing someone through a wall of thorns does? Average 12 damage per trip through the wall. A Wizard is unlikely to survive 4 pushes.
So even if, you have a counter that you think is effective to stop it, you pretty much have to have those counters in every tournament spell book you make. (and we're not talking about a couple cards....you're going to have to commit 1/4 or more of your spell book to counter a good Wall of Thorns player.) Your normal strategy is unlikely to work, since the Wall of Thorns deals damage faster than pretty much any other strategy, it pretty much forces anyone who wants to play in a tournament to have to commit 1/4 or more of their book to "Wall of Thorns defense"....or play Wall themselves. This really sques the meta-game play tournament environment.
Frankly if a particular card causes the every tournament player to have to bring a dozen or more counter strategy cards to accommodate that one strategy caused by that card, it is a bad card to have in the game. It should be banned or nerfed. If the card wording were just something like "This Card treats Mages as level 2" Something like that would just fix the whole problem. But it is a problem to be sure.