One thing that really makes Monster Hunter unique is that the game incorporates a timer in all of its quests. The traditional time for you to complete a quest has always been 50 minutes, with some being 35 minutes (real life time).
According to Monster Hunter lore, being a monster hunter is a competitive career, and the Guild (which is basically the quest giver) tests its hunters to see if they are the right fit for the job. This also explains why quests become more and more difficult while the time limit remains the same.
Now, that doesn't mean that you should panic right away and say to yourself, "I'll never complete tougher quests if I'm rushing against the clock..." If you come prepared, know what you're doing, and have the experience (which you eventually will from playing the game), you'll definitely be able to complete those quests.
Besides, as you get closer and closer to the time limit, the feeling of adrenaline takes over your body as you try to deliver the final blow, and then brag to your friends how you took down a very tough monster all by yourself! Nothing can beat that feeling.