To nip this flame in the bud. Diminishing XP won't return, but XP values should be linked to critters stats.
Are we expected to see raised levelling time?
Just to make it clear:
Currently only thing levelling offers is waste of resources and time.
Encounters are either way too risky or way to trivial, there's no way you'd just kill robbers or other bandits to get experience, as this is said to be compensated with loot you recieve ( so in fact instead of xp you get almost always melee and small guns, useful only part of population, and usualy cannot even be traded for what you need ).
Molerats provided quite quick way to level, due to their critical hit tables and relative low damage - meaning you were guaranted quite fast way to level, albeit really unexciting and without just risk of being PKed.
On other hand if one would level up on aliens and perhaps some other FEV filth, they could get radiated, or just have to run away do to monsters being scaled quite up - getting radiated is definetly not fun at all, because of persistence of the affliction, you must cure it manually, while this status also makes it much more harder to get treatment.
While I could also debate about resources (as usual, only SG and melee tier 0 is obtainable without mining, and thus limiting yourself to specific areas ) there is also little clues about destination difficulty and benefits.
For level one guy with a masuer or something he might obtain easily, rats and scorpions will probably be small challenge in TB (but can be still quite dangerous in realtime) and will get fine experience (40 rats to level up is fine), however he must be well placed so he wouldn't run in anything else (like marauders for NCR spawns, or crazies for HUB/VC spawn). But sadly this is when fun ends, EW and BG can't just spend their ammo on critters of this size.
There's quite a bunch of stuff to suggest, while not being insane task ( like adding clues to quest giver locations ). And I'd definetly love to see 'shit shoveling' to be gone, or at least not recommended way to get a level - going from 1 to 2 requires 100 actions, which is completly ridiculous with current spawn rate and Brahmin AI ( they're not moving from their dungs, making players shove them, complication already tedious task ).