Ok so the title is maybe a little bit overblown. I still want to talk about sort of the central thing you need to think about when you're playing a PvE hunter now. You're on a see-saw, with "getting focus capped" at one end and "getting focus starved" on the other. Let me explain.
I'll start with the first side of the see-saw: getting focus capped. What this means is that, once a boss fight starts, you never want to be at 100 focus. If you think about your new resource, this will make a lot of intuitive sense. The 4.01 hunter works as hard as she or he can to convert focus into damage: that's how we kill the boss. You're constantly regenerating focus at a set rate, even if you're not casting steadyshots. If you spend any time at 100 focus, then every tick of focus you regenerate past that is focus that's gone forever: you will never be able to turn it into damage, and that is why it's so important to never be focus capped.
The other side of the see-saw, though, is focus starvation. To understand what this means, just take a quick look at all three specs: all three of them have one ability that costs a ton of focus and has a short-duration cooldown. BM has Kill Command, MM has Chimera Shot, and SV has Explosive Shot. You'll notice that two of those have 6-second cooldowns, and chimera shot has cooldown reduction from talents. So here's the thing: you always want these abilities on cooldown. Each spec has other things to monitor and think about, of course, but the first thing you'll want to practice and get a hang of is always being able to hit these abilities the very second they become available. If you've been firing too many Arcane Shots, though, you'll find that you don't have enough focus to cast one of the big abilities as soon as it becomes available. This is focus starvation.
So if you've been feeling lost since the patch, the first thing you'll want to practice is just heading over to a dummy and using nothing but steady shot and one of those focus dumps. You'll find before too long that if you cast nothing but steady shot and chimera shot (or whatever), you'll spend a lot of time at 100 focus. So then you can try adding in some arcane shots to burn some of that focus, or keeping up serpent sting as survival, or whatever. The more you practice, the more you'll find that you're getting a sense for the point of balance between focus surfeit and starvation.