If you're new, you get matched up against other new players I'm sure. I played the free Arena (after unlocking all the decks in practice) and got 7 wins with a below average mage deck (I'd rank it below average because I wasn't sure how good some of the cards were, and they turned out being pretty bad). Now, I have a good amount of Magic the Gathering experience, so I had an edge, but most of my wins happened earlier than they should have due to players just making bad plays. Not attacking cards in the proper order or not really playing cards in the best order.
Of all the games I lost, all 3 were to shamans that got early "big" cards and windfury on them. That really eats you up if you don't have a ton of spells to kill or CC them.
Also, the practice decks are awful. It is painful to play as anything because of how bad those basic decks are. =(