Now, Blizzard is already aware of this problem. I can understand the struggle they're having with implementing more monsters in Act I and II (IV should be an easy fix being you're battling in the Heavens. There is no excuse why there's less demons there than in Act III). Instead of increasing the amount of demons in Act I and II, make the useless random dungeons (ex/ Decaying Crypt, Lost Mine, Sirocco Caverns, The Ruins) useful.
How would you go about making these dungeons viable you might ask?
Well, this is a DUNGEON CRAWLER, instead of them ending at level 3, make them end at level 20! Each level can be more dense, more DENSELY overwhelming, and drop more items. :D
I enjoy farming, however, I don't enjoy hearing the same dialogue/story everytime I do so!
What made D2 awesome (for me) was farming bosses. Well, I had an epiphany a couple months ago, D3 will never be D2, but that's okay... D3 has A LOT of potential. I don't play because of the end game (or lack there of), I play because it's fun to slay demons and watch their bodies explode across my screen. If they implement something like I stated above and include ladders/races as well, then I'd see myself playing this game for a very long time!