I don't play magic but someone said there's a mechanic that interrupts the opponent's turn on certain actions?
Here's the slightly inaccurate TL;DR. Spells, creatures and abilities in MTG can be "normal speed", or they can be "instant". Normal speed cards are played roughly like in Hearthstone - on your turn, before or after you attack. Instant cards, however, can be played at any time, in response to any other action. If you play an instant in response to another action, the instant occurs first (but another instant can be played in response to that, etc).
This allows for board position to change at any time, you never know if an action is safe, unless you know what your opponent has in hand. It massively increases the complexity of any decision.
The downside is that it bogs the game down. After every single action, you must ask your opponent if he wants to respond before you proceed.
Like Mist said, Hearthstone devs have obviously decided that they would rather have a fast paced, easier game. That's fine, the game is fun. However, through that decision, the complexity is reduced so much that the game becomes less suited as a professional competitive game, since there are far fewer opportunities to outplay your opponent.
Note: The less obvious downside of Hearthstone's design decision is that it limits how powerful your cards can be in certain respects, and thus it limits deck variety. Unleash the Hounds, for example. The card is interesting and allows for a fun archetype to exist, but it will be nerfed because there is no good way to counter a single turn kill in Hearthstone. In Magic, instant abilities would allow you to do that. For example, Blue could counterspell the key spells, Red could cast instant damage spells to neuter the attack, White and Green could play "no damage this turn" spells, Black could make you discard key components of the combo. This would allow the deck type to exist, thus causing a broader and more interesting metagame.
to keep me alive till I could use them.
