There is no evidence, aside from fan (mis)interpretation, that Luke's lightning is special, or that it is "instakill".
It's merely higher-powered lightning -- nothing more, nothing less -- that, in this situation, was enough to take out a particular enemy combatant.
If I was to fire a blaster at you, and it killed you instantly, would you say that it must be as powerful as a turbolaser bolt, because that could kill you instantly too?
On Luke vs. Lumiya: People (including myself, just from hearing about him, at times) tend to ascribe deity-like traits to the powers and abilities of NJO/Post-NJO Luke.
The reason for this is akin to the reason people say things like "nihilus > all" -- they've been seen to do really awesome things, above anyone else, and so they take on these new heights, and so people, not being able to quantify the power gap (if there is one), simply label certain chaps as "uber" and regard their attacks as like magical fireballs, slaughtering everything and everyone without end, except if interfered by by another uber person.
It's obvious that Luke isn't all-powerful, since Lumiya attacked him, while he was unfocused on the problem at hand, and managed to do some degree of damage.
Oh, and saying that "Luke had no intention of harming Lumiya" is irrelevent. Darth Vader in ROTJ didn't want to kill Luke, but that doesn't mean that he would have let himself get kicked around the room because of that -- such an argument makes no sense at all. It's obvious that he wasn't putting all his strength and will into the fight -- that he was unfocused, unsure, etc. -- but he should still have been able to at least hold his own for just a few minutes.
Just as Vader failed in ROTJ to protect himself against his maddened son's wild attack, NJO Luke failed to protect himself from Lumiya's attack. It's obvious that Luke is *not* all-powerful, and this, in fact, shows a good amount of mental weakness on his part, that other Jedi might not possess. Do you think Yoda or Obi-Wan would be confused about killing Lumiya? They'd just run her through, wipe the dust off their robes, and move on. While his compassion may be one of his greatest overall strengths, in this particular fight, it gave support to his enemy.