iApple Products

Yeah, yeah, those huge beautiful screens, cameras that will make you look like the next Kubrick and curved edges that make your hands tremble to touch them. We've all heard it: The iPhone 6 and the iPhone 6 Plus are the biggest—and best—iPhones yet.

Except what about those of us who can't justify an upgrade or aren't eligible for one right now? To those people, all of this sweet iPhone 6 talk sounds worse than a neighbor bragging about his new Tesla.

It turns out iPhone 4S, 5, 5S and 5C owners will end up with a basically new phone when they upgrade to iOS 8, a free download available on Wednesday. I've been testing the new software on older iPhones and the new handsets and I can say it isn't the size of the screen but what's on it that is pushing the iPhone ahead.

Apple says this is the biggest iOS feature release ever, but you wouldn't know it on the surface. Underneath the translucent menus and bright icons, new functions have been added to the apps you use the most, and some of my basic iPhone issues have been fixed. (Annoyingly, though, others have not.)

Beyond that, Apple has begun to loosen its grip on its platform, opening up the way you, your apps and phone interact with everything around you.

The New and the Improved

By and large, iOS 8 is focused on making Apple's core apps better and getting them to play well with others.

When it comes to operating-system fixes, one of the most welcome addresses a pet peeve I had with the Mail app. Instead of breaking into a sweat every time I have to copy and paste text between two email messages, I can now easily drag my new message to the bottom of the screen, glance at my inbox or another message and then jump back to the draft message.

There are other new email tricks, too, such as scanning messages for contact information to quickly add to the address book. Similar to the popular third-party Mailbox app, you can now triage your inbox by swiping on a message to delete, archive or flag it.

Apple's iMessage is now packed with features that take direct aim at popular messaging apps like WhatsApp and Google's Hangouts. You can share your location, easily leave and mute a group iMessage chain and send quick audio and video messages, which expire two minutes after they're played. It's added more spontaneity to my communication, though Apple should make it clearer that the messages go away. (For media you want to save, there is a "keep" button.)

Of all the iOS 8 fixes, I'm downright giddy about what's happening to the keyboard. For years, the iPhone keyboard was stuck in the past, while Android allowed for alternative layouts and speed-typing tricks.

Apple's fresh QuickType keyboard suggests the next word you might type based on the context of your conversation, the person you are messaging and what app you are in. For me, it has replaced autocorrect. While predictive typing speeds things up, I've gotten even faster using third-party keyboards that Apple now lets you download from the App Store. I've been testing SwiftKey and Swype, both of which guess what word you want to type as you drag your finger around the keyboard. So far, SwiftKey seems to have learned my typing behavior better.

The keyboards are just one example of how apps can now integrate and talk to Apple's hardware and software. Even the iPhone's Touch ID thumbprint reader now talks with other apps. I tested a beta version of my favorite password manager, Dashlane, that can use my thumbprint as a master password and fills in usernames and passwords in the mobile Web browser automatically.

Listen to the Body, Talk to the Mac

Apple finally gets into the exploding field of fitness tracking with iOS 8. Everyone who updates gets the Health app. It's meant to be a central dashboard of all your fitness and health data, so you no longer have to look in one app to see calories consumed and another to see calories burned.

Both the iPhone 6 and iPhone 5S did a good job of tracking my steps and distance throughout the day. The new motion co-processor in the iPhone 6 went a step further, tracking my climbing up stairs. Soon third-party fitness trackers like Jawbone Up24 and apps like MyFitnessPal will report data to the Health app.

Apple's Health app is nowhere near as compelling as Jawbone's or Fitbit's. It feels more like a medical report than a daily destination. It has the potential to be incredibly powerful, especially once it links up with the Apple Watch and third-party apps to analyze the collective data, but for now it's unfriendly and confusing.

While an iPhone running iOS 8 connects with more devices than ever, I'm most excited about how it connects to my computer. The next Mac operating system, called OS X Yosemite, enables you to accept iPhone calls right on your Mac and "hand off" emails, webpages and messages to your computer or phone.

I tested a few of the features with the beta version of the Mac software. When my iPhone was connected to the same Wi-Fi network as my MacBook, I was able to pick up an incoming cellular call from my mom right on my laptop, which sounded fine.

Even more useful was the email hand-off. I began an email to my colleague in Apple's iPhone Mail app, then finished it on my laptop by clicking a small icon on my MacBook's screen. It worked the other way, too: When I pulled up a restaurant website on my laptop, a Safari icon popped up on the lock screen of my iPhone. With a swipe, the website loaded on my phone.

What Still Needs Work

All of those features make iOS 8 more mature than ever before, yet there are places where it continues to lag behind Google's Android and even Microsoft's Windows Phone.

Apple's Maps is still missing transit directions and many points of interest. As soon as I got the iPhone 6, I downloaded Google Maps so I could figure out what train to take back to my hotel in San Francisco.

Siri is also stuck in elementary school, while Google Now and Cortana graduate upward. Apple's personal assistant now identifies songs playing around you and will soon help turn on the lights or adjust the thermostat, but you have to prompt her. The other two anticipate what I need to know (traffic on the way to my next meeting, my favorite teams' scores) before I ask.

The new iCloud Drive is a big step in the right direction, letting you drag PDFs, Word docs and other files into a folder on your Mac or PC then access them on your iPhone, iPad or even the Web. But iCloud still needs an owner's manual to navigate. And iCloud Photo Library, which will roll out in October, is confusingly an entirely separate service.

The operating system's organization feels stale, too. The information and apps I need the most are buried. While the Today screen will soon support third-party widgets, I wasn't able to test it and besides, it feels a little out of the way. There's no easy way to just peek into an app from the home screen, or personalize a device, a la Android and Windows Phone.

Speaking of personalization, why can't I personalize iOS's pop-up Control Center with tools I need the most, or at least provide a link straight to my settings?

I'll probably always have gripes, but iOS 8 pulls out ahead of the competition by leveraging the greatest strengths of the iPhone: its apps and ecosystem. After you tap the Update button, those small perks and fixes will make the most difference.

Over time, as software developers release apps that talk to each other, to Apple's hardware and to its new health and home software initiatives, the iPhone will have the power to become something much bigger than it already is—no big screen required.

—Write to Joanna Stern at joanna.stern@wsj.com or follow her on Twitter @JoannaStern.