1) Double-Up needs to cast faster as you build attack speed. It's only fair, Riot. The cast is fine during laning, but after a Phantom Dancer, you have almost no reason to touch Q anymore. Make it cast faster so we can actually animation cancel with it properly. It's basically another auto anyway.
2) Impure Shots takes too long to have a noticeable effect. Vayne gets %MAX HEALTH TRUE DAMAGE in only 3 shots. MF has to steadily stack Impure Shots as it climbs from 10 magic dmg...to 20...to 30...and so on. Takes way too long to only add a *meh* amount of extra damage, and we ALL know that the ins-and-outs of team fighting won't let us be for that long. There needs to be more *umph* in this passive.
3) Activating W should allow her passive to stay on for a least a few seconds...I'm personally thinking 2 seconds. I always say "minions should deactivate Strut. It's basically mobis anyway". However, if she's gonna use W to get away or chase while her only other option is only a slow, a random caster's signature move shouldn't deactivate it immediately after using it. It's unsatisfying and immediately has the reverse effect of what Riot was aiming for. Otherwise, the W change was perfect.
4) Her ult is fking garbage. The idea around the champ is that MF is a run-n-gun sort of champ. Her ult, however, keeps her in one position and hardly does shit outside of picking off people when they're at 200 health...maybe. It just doesn't feel like an *ultimate* move, nor does it have the impact of one. This is partially due to her Impure Shots passive being so...lousy, seeing how it's supposed to synergize with it by adding and applying stacks while you shoot, but since you probably don't have any stacks applied, you aren't getting much bonus damage.
Short story: MF is...relatively fine, in all honesty. The implementation of her kit is out of whack, though. Her ult is straight stupid, her Q needs to scale with attack speed, and her Impure Shots, in all honesty, is out-dated now that most champs use the 3 stack rule, rather than indefinitely adding bits of debuff onto people (like, seriously...8 stacks? The fk?). Maybe she needs a new ult, which would require a rework, but if Riot wanted to, it wouldn't be too hard to fix up MF to be a satisfying champ selection. Perhaps they're already cooking something up for her and a few other adcs for the adc rework after worlds?