Here are some quick thoughts about armour, or rather on the weight/protection part of it. Most of it will probably sound obvious though^^.
As noted above, taking the weight and overall body coverage of armours might help you a long way in defining a solid system. Allowing all kind of characters to wear pretty much wathever type of armour is not a bad thing, but the advantages and disadvantages of a specific type of armour for a specific character build should still be present. A weak but agile warrior should not want to wear heavy plate unless some very good reasons. A strong warrior should want to wear plate unless having to sprint or swim or whatever good reason to not have 50kg of metal on your back.
That's pretty obvious stuff, but on that basis, it might be interesting to make the character's ability to move and fight differ a bit. For example, the same warrior wouldn't move nor fight in the same way wearing leather or plate, whatever his strenght is. Gameplay wise, this could be rendered not only as slower execution speed, but also access/advantages to special moves (quick strikes in leather, heavy ones while being kind of reckless but ok thanks to the plates)... I have been practicing Japanese swordmanship (not kendô, the real stuff ) for 10 years, and the techniques vary a lot wether they were meant to be executed wearing armour like during the battles of the Warring States, or without like during the Edo period. The former are more wide moves and big strikes, the latter, more close combat style with small and souple moves.
Another interesting point would be to make the critical strike rate change depending on the type of coverage an armour gives, plate being close to 100% and leather to 50%. It should be more difficult to strike someone critically when he's mostly covered in armour, without having 100% protection of course (plate having weaknesses at the articulations...which are the spots aimed by the techniques against armour in my sword school).
To sum it up: weight influencing movement and techniques - armour types influencing critical rates. Those are my two cents.