One issue that arises, particularly with spellcasters, is whether when - even mid-battle - he gains rank, he immediately has the use of the skills or spells acquired at that next rank at his disposal. One could either say that the PC has to go away and practice these new skills (usually, after a particular adventure) to be able to make use of them or, on the contrary, argue that he has already been researching / practicing these skills, and now feels 'ready' to use them. This applies more so to the skills argument, whereas for spells it could also be argued that some mystical transaction occurs upon the gaining of certain experience in combat that 'unlocks' the ability to use the greater spells.