I have a 21 fury and a 30 defiler, I love them both, i couldnt favor 1 over the other, i don't see the same probs with the ward's/debuffs stated here, infact when on my fury i sometimes wish i had my defiler's buffs.
Ya'll need to learn how to use HO's, not specifically in 4-6 peep groups cause it can get messy (but i find in 4-6 peep groups your issues with being solo healer isnt sush a burden as you should have enuff dps to finish most things off quicker than you really need to heal, so i figure most ppl's complaints about their debuffs/healing/wards are all about soloing and small groups with lil DPS) I duo alot with my zerker gf on my defiler, now all we pretty much do after ive warded her and debuffed the mob is combined HO's, saves so much power i can finish an encounter even 2-3 mob white encounters with 1/4 - 1/2 power pool left. Not only can you do massive damage together on combined HO's but with defiler/zerker (think it might be all fighter/all priest) combined HO's i can heal her for upto 750 which 80% of the time complete heals her that same effect adds a small HoT and a ward for a short time.
I dont believe that there isnt things that need to be fixed on mystic/defiler class but you need to learn how to play it to its best ability, ive never dreaded being solo healer, could mostly be due to i trust my tank. Tho I believe most of the time you are only as good as your tank, if your tank sucks no matter how fast your heals are you cant keep them alive.......that being said, when it all comes down to it, you need to choose ur battles that best suits your abilities.