I hope he was preferring to units. All unlockable achievments should give exactly ZERO in-game mechanical bonuses ... thats just silly. Make someone be able to play the game more easily because they have played enough? pure skill should determine that. Now ... on a campaign, if its played from one map to the next (although I pray to Jebus that the entire campaign is on a single world-map)
So ... as to my point I HOPE he is talkin bout' leveling on a soldier-to-soldier basis. Of course, benefits of levelling should probably scale to how difficult it is to level up ... but here are some rough ideas.
For most basic soldiers (not a sovereign, hero, or family member)
minor stat gain -every two levels
minor trait -every level (cycling between 4 different traits which slowly stack)
major stat gain- every four levels (boost to the minor stat gain)
major trait- every 3 levels (could either be a boost to the minor trait, or a weapon-specific trait AKA wide sweep, or greater phalanx)
so lv 1 = minor trait, lv 2 = minor trait and minor stat, lv 3 = major trait, lv 4 = minor trait and major stat, and so on.
Then, for A Sovereign/hero/family member, all levels will give major stat gains, skill points (leadership, ect), and every few(3 or 4?) levels will gain a special ability or activatable mode for the battlefield (like crushing blow, bezerk, flurry of arrows, ect) ... selectable. Also, it seems to be fairly easy to equip your hero with whatever you have available (not considering magical artifacts) ... so abilities will be selectable from a list, based upon which weapon you wish to focus on (perhaps armor too, unless armor is kept super-simplified). Perhaps only 5 possible abilities and 5 possible Activatable modes. Maybe less. All gains for Sovereign will be more than that of heroes and royals, of course, and also Sovereign will be able to place skill points into places heroes couldn't even dream of. (Sovereigns are not only better at royals and heroes at what they do, but he can do so, so much more)