Unfortunately XP penalties are imposed for grouping, even with players of your own level.
The XP slowdown was obviously done in anticipation of the heavily advertised Allied Races, the unlocking of which requires playing old content and the playing of which requires leveling through old content.
Blizzard’s continuing refusal to merge servers and its preference for CRZ “matching” means that actual WoW communities will never again be what they once were.
You can’t guild or trade with transient players; power leveling is dead; dungeons are unrewarding and take far too long; grouping is penalized.
What has happened is that people are not bothering to level alts. They have quickly found that playing at max level is an endless and unfun, unrewarding grind.
Servers continue to be ghost towns; guild rosters are dark; friends lists are dark and yet, there hasn’t been a single effort to respond to any of these complaints.
The upcoming “level boost” in 8.1 is a joke; for the first 40 levels the level cap actually is raised! Then, gradually reduced until at level 60 or so, you receive the benefit of the 40% Xp cap reduction until about level 75-80ish. Then the XP cap gradually rises again.
What was needed was a scale/leveling toggle (yes it can be done, see FF14, SWTOR or GW2); OR raised XP rewards as well, not only for questing but for every other in-game activity for which experience is rewarded.
I do not have high hopes, but I still have some hope that someday, this game will be fun again.