Yes, that's right, I've gotten my 'Demon Hunter' to level 22, strictly by grinding on Demons!
Wyndampryce on the Armory
This screenshot was taken after leveling to 22 and heading back to Stormwind to train.
That's 1094 mobs killed, 1094 Demons! (and 1 Deep Run Rat!)
To update on Wesley(his first name), he is a human rogue. At level 1 I ran him to Teldrassil and leveled to 6 on the demons in the Night Elf newb area. Then to level 10 on the demons in the cave at Dolonar in Teldrassil. Much of level 11 was exploring XP. Then I moved him to the demons on Bloodmyst Isle, the ones that are level 12-15, southwest of the main town. At around level 15, I moved him to the higher level demons in the north of the island. Between this and XP from exploring has gotten him to 22. The next demons are in Ashenvale starting around levels 23-24.
He has fully explored most of the zones his level and lower(with the exception of the Blood Elf areas). His herbalism is 154 and inscription is 142. He has lucked out and managed to get both the Sinister Squashling from Hallow's End and the Peddlefeet from Love is in the Air!
I generally let him rest in an Inn for a week to build up rested experience until at least the end of the current level. So far each level then has taken about an hour to gain. I know this will take longer and longer as the levels get higher. And will also change depending on the difference in level of the character and the mobs. The first few levels at the 16-18 Satyrs in Bloodmyst took an hour, with stops every so often to bandage or eat. The last few levels there went very quickly to the point where I had almost cleaned out the whole camp and had to wait for respawns as all mobs were green levels. The next group in Ashenvale will go slowly again due needing to stealth more and stop between fights, but the higher xp/mob makes up the difference, so all in all it comes out about the same in time spent/xp gained.
Also, he has gained quite a lot of money for a character his level simply by getting money dropped by mobs, selling every gray/white item they drop to vendors, and selling stack after stack of linen and wool on the auction house. A few glyphs have sold decently as well. He already has the money for his mount when he hits level 30-as well as plenty left over for snapping up a few bargains on the AH for gear when I can find them.
This has been an interesting expirement so far, and I look forward to leveling him up higher-though there aren't many demons in Northrend should I get him that high. Things may have to change at that point. But I will cross that bridge if I ever get to it!