The Problem That is Lux
WARNING! This post is my opinion. For those of you who do not know an opinion is something that is not wrong (there are no facts refuting it). That being said my opinion is necessarily not right, and neither is yours. While I would love to hear your opinion just because it may be different than mine does not mean it is right, mine is wrong, or vice versa.
TLDR; I'm assuming you are here to see how I would nerf her. My nerfs are in Italics, so they should be easy enough to find. Keep in mind that if you do not read everything I may have said things that you comment about. To avoid looking like an idiot and bringing up things I do talk about (you know saying the same things I address) if you are only talking about/read my suggested nerfs then only talk about the nerfs. I do talk about her damage, but not as a nerf, and that is the very last section. Any sort of feed back is greatly appreciated.
Lux is a VERY long range poke mage with the burst of syndra, the long ranged waveclear of Xerath, good enough CC to rival Leona, and the mechanical requirement of a fed Katarina. All jokes aside I think Lux is in a very good place, and overall she doesn't need a huge nerf (just a very small one).
Her passive while adding a lot of damage is not a problem because you need to keep in mind her auto attack range is only 550. It's the same amount of range most ad carries have (unlike ad carries she probably won't do much damage to the front line so she has to walk up to the frontline to use her autos on the back line putting her in a very unsafe position) unless you want to use her ult which can be avoided. Maybe reduce the time that a champion is marked, but it doesn't really last any longer than any other mark.
Her Q can snare two enemies for up to two seconds, and can be very annoying. However, the hit box is pretty skinny making it a relatively easy skill shot to avoid. If you do get hit and are a squishy champion odds are she is going to follow up with her E and her Ult. As an ad champion getting a QSS will break the root allowing you to dodge the ult, and as an ap champion you can use zhonya's while rooted to avoid the damage of her ult (unless the lux is super fed and you are far behind her E Q combo typically isn't enough to kill you).
Her W is very annoying as the shield late game shields her and all of the allies hit for about 500 late game (I don't feel like doing math, it is probably more, but this is just a good middle ground). It refreshes the shield or reapplies it to any allied champion it hits and lux on it's way back. While the shield is annoying I don't think it really has all that much of an effect outside of laning phase because of two things. First it is not likely that the shield really hits any one on her team in a chaotic team fight (most lux in my experience are throwing it out the direction the enemy is because they are trying to deal damage and shield themselves (it is much slower to try to move your mouse back and forth between your enemies and allies trying to deal damage and shield). Second Lux herself is very squishy, and while the shield helps to mitigate the damage she takes so he has a chance of survival in the end it is no more than the shield that a lulu, jana, or other shielding champs can provide. The biggest problem is that it shields twice, and yes this can be a game changer (in my experience it isn't because either lux has died before it returns to her, she still takes too much damage after the fact, or she is too far away to take damage), but it really has no greater impact than a support with a shield on a much lower cooldown.
If this were to be nerfed (I don't think it needs to be) then a possible option would be on it's way back it doesn't shield allies, and only shields lux for half of what it originally shielded her for stacking on the original shield if lux has taken damage before it returns to her.
Her E is the only part of her kit that I think needs a nerf. It does a very large amount of AoE damage, slows, and is damn near unavoidable. Lux was really bad when her E could not be reactivated immediately, and became very good when this change was made. The problem with this immediate reactivation is that it makes it damn near impossible to dodge. Lux throws out her Q and unless you were already walking away from where it would be you will most likely be taking damage. This makes laning against Lux hell on earth because most people stand behind the caster minions she gets to cs while harassing or she zones you off of the wave and you miss out.
The problem with putting a delay on the reactivation is that she will go back to being as bad as she was before she obtained immediate activation. This is my solution. Leave the range of the slow, and AoE damage the same. Make it so regardless of when lux casts her E if you are in the current slowing zone (the little death circle) you will be slowed. Leave her ability to immediately reactivate her E to deal damage the same. However, make the range that she can deal damage start at a smaller point within the circle and charge up to where it will deal damage across the entire circle. For example let's say the area of her E has range of 1000 (I know it's not that would be insane remember this is just an example). If you are in this area you are slowed immediately (like it is now), but immediately reactivating her E only deals damage at an initial range of say 500, and to deal damage across the entire area she has to let it charge up to 1000 over time (keeping the ability to reactivate at anytime leaving it possible for her to deal damage at a range of say 700, pretty similar to the way varus Q works but in a circle, not a line, and lux doesn't have to charge the damage). Causing her E to work in this way would cause the damage to be avoidable, without reducing her overall wave clear and possible AoE damage.
Her R is perfectly fine the way it is, and while some people may think that it is stupid that it can activate her passive keep in mind there are ways to avoid her ult (I discussed this earlier), and without it her passive would be completely useless outside of laning phase (she only has an auto attack range of 550 meaning she needs to be on the frontline to auto attack the backline).
I do not talk about her damage in the nerfs because I think her damage is fine overall (maybe very slight reductions here or there, but nothing large enough worth mentioning). You have to keep in mind that the lowest her cooldowns are without cooldown reduction is 10 seconds and with max cooldown reduction (45%) they only get down to 5.5 seconds at the lowest. Meaning if lux does not blow someone up she is useless for over 5 seconds (she needs the high damage).