The matter at hand is as Derek Paxton said concerning the Experience gain increase coming up in 0.85 (as stated in the Changelog):
Increased the xp given from combat slightly
Its a pretty complex formula. Mostly because it relies on other things. The base per monster is the monsters battle rank (which is a big calculation in and of itself) times the GetCombatRatingToExpMultiplier in elemental.defs (the value that 0.85 switches from 0.05 to 0.07). At a minimum units give 1 xp per level.
Battle rank is also modified by any CombatRating modifier on attributes (which is how we tell the threat level and exp systems that particular traits make monsters more or less dangerous).
Once we have the xp value for a particular monster we modify by the amount of champions in the winning army. We also modify by world size (you get less xp on larger maps to balance for the more monsters and battles). Finally we have the ability to set a minimum xp in elementaldefs (which is set to 1) and a maximum xp (which isn't being applied since it is -1).
Im sure there are a few other things, but in general thats how it works.
This means an increase of the multiplier by 2/5 which amounts to an experience bonus of 40% (0.02 of 0.05 is +(2/5)*original value)
Currently there is a bug concerning the level up screen/process:
When there are no more traits to pick up for the hero leveling up you are stuck in the level up screen (major issue).
At the moment this point is easily reached on Huge maps with Dense monsters and Insane world difficulty. The upcoming change will make it even easier to get to this point at which you are basically stuck, meaning you either need to never use your fully leveled hero or concern yourself with the workaround I will describe further down this post.
This needs to be fixed because at that point it is a major concern.
3 ideas I'll throw at you guys right now:
1. Add a bonus that you can always choose at levelup and that is just like the endless research endlessly levelable (e.g. +1 HP as a boost) something you wouldn't take before maxing everything else so to speak.
2. Just disable the levelup screen by the point every trait is taken
3. Allow for the taking of a second class somewhere at a specific level (e.g. 20) that would only move the problem back a few levels but it is nevertheless a nice idea.
Anyways this needs fixing (I am repeating myself I think).
The current (ingame) workaround I discovered is:
1. Click your heroes portrait in the levelup screen (you are taken to your heroes overview page)
2. Double press "Esc" fast (much like double clicking). You should be in the pause menu now (if not, back to step 1.)
3. Click "Save Game" (nothing should happen).
4. Click "Back to Game" (the save game screen should show up if not back go back to step 1.)
5. Save the game wherever you want.
6. Return to the game by using either "Esc" or "Back to game" just like you would regularly after saving
7. The levelup screen should be gone (if not back to step 1.)
8. Either close the game completely and restart then load your saved game (everything should work fine until the next levelup then go back to step 1.)
OR
9. Optional : Try hitting the "Turn" button (the hourglass should appear, but nothing will happen in the next minutes or so)
10. Try reloading the new save without restarting the game (if you did 9. the hourglass will show throughout the whole reload process)
9.+10. is not recommend -> generally your game will crash (you can then reload normally).
