There are really two different level ranges.
Froob: Anything over 150. (Means: Read my thread in the
other forum for my thoughts on that.
)
Sloob: Starting about 130 to about 160. 130 is when Ely hecks become not as good of XP. People generally migrate up to Ado, but due to the way Inf missions work at 160 they move to Inf, meaning that the people stuck in ado is about 30 level ranges and the XP isn't great. Ado hecks are a bit too hard to kill for a team of 130-160 players.
Improvements: Reduce the difficulty of Pen missions for the same reward. Pen missions are almost (if not) as difficult as Inf missions with far less reward. This
should actually provide a good leveling range for 150-180ish.
Consider level locking Inf in some way/shape or form. A level lock on Inf boots, perhaps. Level 160 players
should not be in Inf. 180+ I could understand, really 200+ would be better. Inf is a great place to level for 200+, and should be left as is.
Full Expansion: The issue with leveling from 200+ has been dealt with in LoX. LoX is actually a
great place to get XP/SK, and provides an alternative to inf missions (which I swore to never do again after doing ten billion of them on my main).
Full expansion players still have the same issue as sloobs with a horrible time leveling from about 130 to 160. See all my comments for sloobs and apply here.
So to sum up, froobs from 150+, sloobs+ from 130-160. Technogen has a lot of good thoughts (all of his comments on Ado missions are absolute money).