You've got a good baseline here. I think what it is missing the most is information about what each part actually does. Some kind of info box that pops up when you hover over a part and describes the effect it has and some basic stats such as cooldown time/damage value could go a long way.
Also, the drag and drop controls don't feel very responsive. It's mostly a limitation of the web interface. I think a double-click to equip or even just an equip/unequip/merge button on each part would be much better.