The arena feels pretty much like normal Gwent, which is great as it implies that Gwent has enough of an identity too feel like itself even if some of the basic rules are changed so much, but at the other side it is a bit of a shame, that there isn't such a big difference.
What makes the main difference for me is the time. Arena is a lot slower as noone nows there deck such good or the one of the opponent and eveery turn just takes ages. That is ok, when it is your turn, but waiting for the opponent can get really annoying, as there are so much less things to do. Without synergie you can't plan far ahead and you can neither count cards.
I belive giving information(how many cards of which faction or how many cards of which colour) about the opponent would make it a bit more interesting, as it is possible to plan further ahead.
Concerning the deck building, I probably hadn't a really bad deck, but I find that the difference between bronze, silver and gold is much less then expected. There a lot more bronze units you can play without any setup than golds, so if you have only gold units, you will have to play some of them when it is not optimal. Also a lot of Golds and Silvers rely on the player having bronzes. Therefore, I only once felt that I had lost a game due to the number of golds and even then it was more which golds the enemy had then the number of them.