Previously, the U.S. carriers were basically king-makers selecting which devices were available for sale and what features were enabled on them — to the point where I knew a couple of people who imported phones from Europe to get something which either wasn't available from a U.S. carrier or was restricted to one they didn't wish to be a customer of for some reason (e.g. poor coverage), and it wasn't uncommon to hear about people reflashing phone firmware to remove things like the carrier's wallpaper or preinstalled apps which the user was prevented from deleting. I refreshed a phone simply to change the audio codec settings because Cingular had restricted it to the lowest-bitrate which was painfully garbled.

It's actually kind of hard to articulate how bad things were in the early-mid 2000s with respect to carriers all trying to push their own garbage and monetize absolutely everything possible.

2 of those aren't phones, did you mean something else?

On the other three, that's kind of like saying that the IBM Selectric blazed the trail for the PC — not wrong, but minimizing how much room there was still to go. If you used a Blackberry, it was good at message, tolerable for email, and bad at everything else. The Treos had some nice ideas but were agonizing slow, PalmOS was unstable, the software selection was limited, and the UI was … uninspired. Windows Mobile was at least stable but held back by mediocre hardware, and it had the lowest-common denominator problem that developers had to deal with the same OS running on a wide range of hardware and thus apps weren't usually tailored to work with the hardware.

The major trailblazing things added by the iPhone were the first touchscreen UI which was often better than a desktop UI — the Palm-style touchscreens with a stylus were much clumsier – and a desktop-grade web browser. Yes, other devices had mobile browsers earlier but they had significant compatibility issues and the UIs were uniformly clumsy — I briefly had a work Blackberry which was one of the last non-Android devices they made and despite coming out later, it was worse that an iPhone 1 in every way except messaging, and that's only if you excluded the App Store because that wasn't in the first release.

One of the big things which people underestimated was how much bigger the iPhone screen felt: when you were used to having something that size which was half controls, the iPhone had significantly more usable space (at higher quality than the displays common on many phones at the time, too) and the touchscreen keyboard could adapt to the context so it was much easier to use than the various systems like A9 which some phone manufacturers had invented. One nice side-effect of this is that it made localization better since the manufacturer didn't need to ship different hardware for different languages and multilingual users could just switch.

The final big thing was that breaking the carrier roadblock wasn't just about cheaper data. It also meant that the carriers didn't disable features based on marketing plans or try to force you to use some terrible app or services they got a kickback on (iOS 1 didn't support other apps but the bundled ones were quite good compared to the field and the App Store shipped shortly afterwards, which finally unleashed the mobile development industry which had been held back by the carriers).