Razorfen Kraul is 35-45 instance, much like Scarlet Monastery Cathedral part.
On this whole issue though, I find that there are often mobs that are easier to kill and give better experience then the ones in instances. For one, bosses in instances can take up a lot of time/effort (phat lewt aside), which you could have spent leveling (if you are going that far to just level). Also, there are many mobs that can be killed relatively fast by a particular class, with little danger to oneself and with good experience rewards. For example, the elite whelps in Badlands. If you can prevent them from casting fireballs most of the time, you can kill them over and over again for some very good and fast experience. It's boring as hell, but it's fastest way to level at 40-45 if you can handle them.
Still, I wouldn't discard quests completely - some quests are so east and fast to do, while giving relatively high experience, that it's silly not to do them.
On this whole issue though, I find that there are often mobs that are easier to kill and give better experience then the ones in instances. For one, bosses in instances can take up a lot of time/effort (phat lewt aside), which you could have spent leveling (if you are going that far to just level). Also, there are many mobs that can be killed relatively fast by a particular class, with little danger to oneself and with good experience rewards. For example, the elite whelps in Badlands. If you can prevent them from casting fireballs most of the time, you can kill them over and over again for some very good and fast experience. It's boring as hell, but it's fastest way to level at 40-45 if you can handle them.
Still, I wouldn't discard quests completely - some quests are so east and fast to do, while giving relatively high experience, that it's silly not to do them.