The people who are exclusively questing and have skipped the collection quests are running out of quests.
Just looking at these forums for that conclusion.

So, either do some of the collection quests, as many of those then open up kill quests.
Or do the kill quests and skip the collection quests and get your remaining exp's elsewhere.
The two options are mindlessly grind, preferably adding in herbs, mining nodes and skinning to at least make your toons a lot of gold in the process.
Or run instances; even if you cannot beat a given boss you can clear trash to them and reset. Eventually you'll get bosses down too.

The process of leveling up isn't boxer friendly in this expansion.
But then blizzard designs their games with the intent that 99% of the player base will end up spending 99% of their time at maximum level.
So to really experience Wrath, you need to be 80 and see what it has to offer at that level.
The grind up might be a pain, but you'll get there...