Apple usually takes the time they need to release products that work well. They are seldom first, but as John Gruber succinctly put it years ago, they are usually "first to do it right". With their move to minimalist design, they were neither first, nor, unfortunately, did they do it right. So while I'm griping about fundamental usability issues in iOS 7, I may as well let loose on some other problems:

Weather

Have you ever tried reading the forecast when conditions are hazy? White text against a gray background sure looks pretty. Hope your eyes are as sharp as a 6-year-old's, instead of half-blurry as you wake from sleep and try to figure out how to dress for the day.
The weather forecast in Notification Center is now written in conversational language rather than being represented with numbers and iconography. Good thing you weren't trying be quick by using the Notification Center shortcut to check the weather, right? Right?!

Music

Do you play music wirelessly using AirPlay or Bluetooth? Great! Switching to wireless speakers now requires navigating a couple levels deep inside control center, rather than having a toggle directly in the music app. It's far less discoverable and far more cumbersome to use.
Do you rotate your phone sideways to browse your music by album cover? No? Is that because you always see the first alphabetical albums no matter where you are in your music collection? Well, iOS 7 has done nothing to fix this, but the design is now flat! Oh, and Music.app likes to switch into this view much more easily than in previous versions of iOS. Lay your phone on a slightly uneven surface, and chances are Album Wall will take over.

Safari

Did you forget the Reader feature exists? It probably doesn't help that the redesign gave the button the same look and location as a website's favicon. Oh, and it doesn't look like a button.

Reminders

Switching between lists in Reminders requires a tap or swipe up from a narrow area at the bottom of the screen. Careful you don't invoke Control Center instead! You only have a few millimeters of space to get this action right.

Maps

I almost feel bad for kicking Maps while it's still down, but not quite. If I look up a street address without specifying my city, don't show me a location in another state. My phone knows exactly where I'm standing. Use this basic piece of contextual info.

Control Center

Do you use your phone as an alarm clock every day? That shortcut to the Clock app in Control Center sure is handy huh? Wouldn't it be nice if it didn't switch you into the countdown timer tab every time? Apparently you should be timing things more often. Maybe this was an internal shortcut the design team at Apple used to stick to the deadline that made them ship a major interface overhaul in too little time.
Isn't it great how Apple overloaded the swipe-from-bottom-of-screen gesture on the lock screen? If you try to bring up Control Center you sometimes launch the camera instead. And vice versa.
Speaking of swiping up from the bottom of the screen, turns out Control Center doesn't always like to pop up. Usually when there's a keyboard on screen. Ever tried invoking it from the Messages app and ended up with a string of gibberish letters in the message field instead?

App Switcher

The redesign of the app switcher now lets you see just 3 apps at once on an iPhone, instead of 4. And best not to think about how little you can see on the iPad compared to the old design. The bigger problem is that navigating the app switcher requires far too delicate a touch. It used to be possible to fling that set of apps to the side to see the next set. No matter how much momentum you swiped with, the app switcher would always scroll and stop at the next discrete set of apps. Now there is no stop boundary, so a quick swipe will send 20 icons flying by before the scrolling slows to a stop. So you must slowly, delicately swipe, then tap the screen to stop too many apps from sliding by, then do the same. Tedious. Time consuming. Concentration-heavy.

Global Problems

Animations. Slow, interaction-blocking animations. Turning them off is not a good solution. They provide important context. There is no good reason for them to take so long, or to stop you from interacting with your device until they've finished.

And if you're hungry for more, plenty of others have weighed in as well.