Well, I myself got some experience with making Builder-type maps...
These are the things I find the most immportant:
1. Different unit types with various specialities and some special abilities (Not only thosethat you cast, but shall we say, can get inside a structure to strenghten it, or some bonus passive attack to specific unit type), and there are no overpowered units and, you know, different unit types counter different units, like Spearmen -> Cavalry.
2. Lots of upgrades. Not those upgrades that add +1 attack and whatever, those are booring and not necceseary for Builder maps. I mean upgrades that give you something cardinally new - unlock new technologies, units and abilities. And you see, the bigger the tech tree is in builder map, the better!
3. Lots of stuff to build. various defensive structures, starting from walls, gates and towers, ending with traps and such. Various unit training, upgrade and income structures with various pros and cons, some aura-giving structures, even beautyfications - variety is very immportant in builder maps.