i believe it all depends on class capability on taking down mobs via soloability or not. It depends solely on what kind of gear you have and (for a casual player) what spec you lean. Because for casual players, you tend to lean a certain way spec/gearwise not multiple... once that is done, your grinding solo difficulty could increase 10 fold.
Take for example, me. I am an old old wow player since vanilla since it came out. I remember the grinding you had to do in order to level. it was INSANE... and let's get straight here people, no matter the spec or class back in the day, it was INSANE. So it is easier in SOME aspects. But I believe Blizz has tried to return that difficulty from Vanilla back a little bit to return some things the old gamers have missed for such a long time.
Getting back to myself, I am a resto (main spec, geared), casual playing druid. this means that unlike what i used to be (crazy hardcore raider), i am not geared on 3 different specs to support those other secondary specs like I was able to back in Vanilla, TBC, and WOTLK.
that being said, since I only really have resto gear, trying to be a boomkin makes my solo grinding insanely hard and slow. So I see MANY MANY people's points when they say it takes way to long and burns them out easily.
However, I can see the other side because If you can find a dps or tank player, as a healer, makes it go by fast because of the fact that you can now pull a LOAD of mobs and just heal the tank or the DPS and be fine. But again it isn't every day your buddy is online to do that.
in conclusion, i do not agree that dailies should be the only source of valor gear, i believe it should be there as some gear and enchantments, but not a majority of them. Again i state that i am NOT completely discounting dailies entirely. I really liked the WOTLK daily system. That i believe offered a happy medium for all sorts of people, both hardcore and casual.