My experience:
As you get further you will have to do more quests to level up but some quests are really easy and give good XP so it doesnt take too long. I play "casually" and even I can level at an ok speed. In terms of just killing things for XP, the only time I really had to was at the very end of the core game and the first level(60-61) of TBC. However going through that bit as a DK was different as they can take more damage compared to my main rogue. In the Litch King I haven't had to grind at all.
As you get further you will have to do more quests to level up but some quests are really easy and give good XP so it doesnt take too long. I play "casually" and even I can level at an ok speed. In terms of just killing things for XP, the only time I really had to was at the very end of the core game and the first level(60-61) of TBC. However going through that bit as a DK was different as they can take more damage compared to my main rogue. In the Litch King I haven't had to grind at all.