I've been thinking about game mechanics recently - be it for the card game variety or of the strategy game variety. (And well, of the Swords and Sticks variety - believe it or not, the original Sword Fantasy RPG battle system mechanic might actually be present.) Every time you think about the game - there are times you run into problem with a particular mechanic or method. The problem is usually choosing between simple and complex. Taking the simple route out - sometimes makes things too simple. There is a lot of difficulty with trying make simple mechanics - without sacrificing the depth of a mechanic.
Simple things like adding elements into the game can make the game difficult to play - because there's always the overhanging need to memorise long lists of what's weak to what, and what's effective against what - however, if that list is short enough, like 3 elements - that can simplify things considerably - but the easiest way is really not to fix the mechanic - but to fix the display and implementation of the mechanic.
Elements are fun - but they're something that should be explained outright on the unit or card. Telling the player what element damage it is dealing, what element it is weak against - keeps things simple - without losing depth. It's not really important that the mechanics themselves are simple - but that the players don't have to keep looking back to the manual, rules or whatever it is that will be hell to read through to understand.
It's really keeping things easy for the player. If you had complex rules for the game - especially an online one - make sure there's an easy way to refer to them. Have complex rules for calculating damage? Make sure the player knows how much damage they can deal way before they even order the attack.