I drew a fast picture about how challenged I felt while leveling my Undead Priest as a LFD healer. I consider anything below 3 as only suitable as a tutorial. 5-6 is probably best for most players. Since I already played WoW and MMOs for many years and tend to play them pretty seriously, I prefer 7-8. Anything above 8 is stress for me and 10 is too stressful to be fun.
Keep in mind that this is 100% subjective. It includes feelings of social pressure, expectations and all this stuff. It is also only about the LFD and about being the healer. Before level 15, when you can first use the LFD, it is about the Undead leveling areas.
Curves for tanks, DDs, questing or PvPing can look radically different.