I had to reread it as well to get a better understanding of it. It is a DOT spell now and that is 20 + .15AP per second at level one. This lasts for 3 seconds, meaning that she will deal 60 + .45AP to champions at level 1, and this is doubled to 6 seconds or 120 + .9 AP against monsters. Given the maximum current%HP damage to monsters was 175 damage on the old Q, this will easily outclass it in terms of clears, though not as bursty. The change to DOT is actually an improvement given her core build seems to always include Liandry's; a DOT refreshes the DOT on Liandry's until the original DOT ends after 3 seconds. So there will be more burn from the item due to the damage being DOT now. Also keep in mind that Q will be better against low hp targets by not being based on %currentHP.
Now the bad parts. The ability is 80 mana to cast, so it is not spammable even though it has a 1 second CD (I think this is kinda stupid honestly and it would be equally effective if the CD was 3 seconds). It actually does lower her burst damage so it gives enemies more chances to fight back or run away if she catches them out. It also lowers her damage against high HP targets, but the tradeoff is that it is more effective against low HP targets.
Elise has plenty of power as is that this won't affect her too much. Her damage and burst is a little bit worse with this change, but I don't think it will gut her as a lot of people think it will. I actually wouldn't be too surprised if her winrate stays the same or even increases after 5.16. Anyway, hope that clears things up. Peace.