The buff: Fiora can Q and E crit turrets now.
Here's the thing with Fiora. She pretty much always wins lanes and duels. But no matter how much she wins lane or wins duels she usually doesn't end up winning games. Why? Because she sucks at both splitpushing and teamfighting, two essential elements of fighters of which either is needed to win games.
This solves that issue by making her a good splitpusher. We've actually been saying she needed a buff in either regard for a long time, but all they've been doing is just buffing her raw damage repeatedly, which all of us despise because she's already outrageously strong in terms of dueling and winning lane.
With that said, while yes this is an overall good change for the gameplay design of Fiora, she will undoubtedly need nerfs to counteract all the buffs she got. Because if this goes through without nerfs to her damage, she's going to win lane, win duels and destroy turrets fast as hell.
TL;DR: Good change, but nerf her damage/cooldowns now or revert some of the buffs she got prior to this.