I don't get how Asajj Ventress, a powerful Dark Jedi, never used Force lightning. A Jedi, Darrus Jeht, used it on her, and it's not like it was such a secret. Why didn't she try it?
Force Lightning has a severe cost, it drains the very soul of the person performing it, not to mention possible physical damage on the user. Just because the force wielder is dark side, does not mean they would be willing to sacrifice some of them self unless it is worth while.
There are tons of reasons why one would avoid force lightning (notably they don't like looking like a shriveled hag)
It is most likely that they just found it not worth it, or ineffective for their purposes.
It was thought that very intense use of Force lightning could drain the user physically, and possibly result in severe facial and body deformations. The deformities Palpatine displayed after unleashing a barrage of Force lightning on Mace Windu, only to have it reflected back upon him by the Jedi Master's lightsaber is an example of this.
When Anakin Skywalker redeemed himself and killed the Emperor, his life support circuitry was critically damaged, leaving him a heavy and lifeless quadriplegic.
Besides draining the user's energy, Force lightning had the added drawback of occasionally charring the hand of the person who used it. When Jacen Solo flew into a rage underneath the ruins of the Jedi Temple on Yuuzhan'tar and attacked his Yuuzhan Vong pursuers with Force lightning, he later found that the blasts had burned charred holes in his palm.
Not every dark side user is at a level to be able to use force lightning. Only the most powerful are at that level. That is why Ventress so wanted to be Dooku's student, to learn those techniques which she did not know.
If a force user gets angry enough and if they have the midi-chlorian count for it, then they can generate force lighting at that moment, as Jacen and Jaina solo both did at various times when fighting the Vong.
Otherwise, it's simply a matter of being at that level and possibly knowing the specific techniques to consciously throw the lightning at will as Dooku and Palpatine certainly could.
If you go back and look at the size of the lightning bolts from both Dooku and Palpatine, you'll notice that there is a difference in the size and density of them. Dooku's are smaller and dimmer where as Palpatines are thicker and brighter, denoting a higher level of power and raw power output.
As others mentioned too, there is the draining of that persons appearance, soul and life force or chi when using the dark side at all, not to mention when throwing force lightning. In other books, it was taught that any use of the dark side eventually casuses the splotching and bruising seen in the Emperors appearance. But in ROTS we saw that in palpatines case, it was because Mace was using Vapaad to repulse the dark side lightning and, along with his lightsaber, send it back towards Palpatine.
I personally thought that Palpatines entire human like appearance was a force mask, and that the lightning simply revealed his true visage. I'm not sure if I read that in ROTS or in another book, but I definitely remember that being a possible explanantion.
In other words, his current decrepit appearance IS his normal appearance through the decades of use of the dark side, and his human looking one was simply an illusion. Luke was also able to totally mask his appearance where others could not tell at all who he was. Only other high level force users were able to tell that Luke was using the force to hide his true form.
This is why, IMO, Yoda was looking at Palpatine funny in Attack of the Clones (AOTC) when they were in Palpatines office that time. I think he was perhaps starting to sense or see through what was going on for a second or two. As we know "hard to see, the dark side is", even, apparently, from someone of Yoda's level.
So that's my 2 cents, ha.