I think the only problem currently is that Zed and Lee need to have higher energy costs. I've done the numbers on a few different boards posts, but basically:
Shen's QWE costs 210 energy always.
Kennen's QWE costs 210 energy initially, down to 170 when maxed.
Akali's QWE costs 200 energy initially, down to 160 when maxed.
Lee's QWE costs 150 energy, but each move has a second cast QWE that costs 90 total. He's sort of a weird case because it's so easy to use his energy recovery mechanism.
Now Zed's QWE costs 165 energy initially, down to 125 when maxed. This is a huge outlier and explains why he can spam endlessly. It should be brought more in line with Akali and Kennen.
Lee should probably have a steeper cost on, for example, his Q if he chooses not to dive in after he casts it (maybe it costs 100 to first-cast, but he gets back 20 if he uses it again)... and maybe his W shouldn't be self-castable (free passive procs and lifesteal/spell vamp for no reason)