The most annoying part about mages is how severely they fuck over players for the first 2-3 turns of the game. It's basically impossible to put anything on the board without giving a mage card advantage until you're playing cards with 3-4 health. Anything with 1 just gets pinged outright, and anything with 2 is probably going to trade unfavourably with a 1 attack minion the mage dropped for basically nothing, like a novice engineer or mana wyrm. Even stuff with 3 health is iffy since by then the mage might have a free 2/1 or something on the board, or some other source of free damage like a juggler or incidental damage from a spell like arcane missiles.
That and the fact that the mage's toolkit is SO fucking broad there are no 'correct' plays, except in retrospect. You have no way of knowing whether you should be aggressive, or be wary or board clears, or wary of spot removal, or what kind of traps there are. At least vs say, a druid, I know he has no hard removal so something liike throwing down a giant ASAP is a good plan, or vs a priest 4 attack cards are relatively safe, vs paladins and shamans and druids you should focus on clearing the board so they can't use buffs effectively, etc. But there's no generally effective strategies to use against mages. They can do anything in theory, and you only find out what was left out of their deck after you've made all the plays already.