The large explosive and damaging skills would cost a lot of resources. The more spammy and weak skills would help to generate resources. This could work on any type of scale, but let's divide the skills into 3 categories as an example.
Total mana resources: 100
1. Very damaging skill. Cost 80 mana
2. Medium damaging skill. Cost 5 mana
3. Lightly damaging skill. Generates 5 mana
This is pretty much exactly what they've done. Very few pure dmg skills have CDs. The CDs are almost all on the non-dmg skills - CCs, buffs/debuffs, mvmt, ect. I personally dislike CDs a whole lot too but it is true that it is hard to balance these without the (IMO) inelegant CD solution.