My favorite build with the KE - doesn't rely on the powerful spells from one tree or another, but the cheap stuff from all three. Energy Barrage from the Storm tree, Winters Grasp and Immolate. By alternating between these three I can pretty much be almost constantly be putting out damage at higher levels. They all recycle pretty quick and are low mana cost, so not much delay between hammering your enemy with one spell or another.
So I throw up a barrier and start pumping out damage, the barrier refreshes and stays up almost indefinitely at higher levels because I'm hitting my enemies enough that they can't tear it down before I can rebuild it.
Add in a couple of passives like winter stillness so you regenerate mana faster by standing still.. and you become a real tough cookie really fast. If you want to see some high damage numbers, take the passive from the fire tree that consumes half your barrier to up the damage on fire spells. Immolate becomes more like nuclear strike at higher levels that way. Me I don't like that combo much, I prefer to have a barrier as close to max as possible and keep it there.
By using this build you don't get quick kills on the big enemies, but you do get reliable kills. When I get high enough levels I usually concentrate on the spirit tree and get resurrection going, as it is common for one or two of my party to drop while I'm still going strong and have barely been touched. It is a surprisingly effective build, and in fact I generally build most of my mages with this same basic philosophy KE or not.
The three bread and butter spells, Winters chill, immolate and energy burst. All low cost, quick recycle, easy damage. From there I specialize depending on the mage. But for a KE build.. you become unstoppable pretty quick this way.