Some of the other mods which add stone armor include a slowness status effect when stone armor is worn. This seems to be based on a 'stone is heavy' kind of argument.

I am not completely sold on the idea, as I'm not much for adding negative features purely based on the realism argument. However, I'm not completely happy with the balance of the new armors vs the vanilla versions. So, perhaps some kind of slow effect may be used as a balancing measure to keep the new armors from overwhelming the old ones.

Tutorial: http://www.minecraftforum.net/forums/mapping-and-modding/mapping-and-modding-tutorials/1571540-1-7-x-1-8-forge-making-custom-armor-that-adds

Custom Enchantment Example: https://github.com/SleepyTrousers/EnderIO/tree/master/src/main/java/crazypants/enderio/enchantment

I almost have this done.

Basic idea: No effects by default. If enabled through the config, wood and/or rock armor will provide an armor encumbrance debuff that will reduce the movement speed of the player.

Each level of the effect will reduce movement by 5%.

Each piece of stone armor adds a level to the effect and each piece of wood armor after the first adds a level to the effect. Full wood will cause a level 3 effect and a 15% reduction in movement speed. A full set of stone armor reaches level 4 and 20% speed reduction. Mixing and matching will work as expected. For example, two stone pieces and two wood pieces will be level 3 and 15%.

The formula is basically, number of wood pieces minus 1 plus number of stone pieces.

Doing some testing and possibly some tweaking, and hope to commit the changes some time in the next few days, pushing a release a few days after that.