Things that can't be changed because they're not fixable in software, but will probably be changed in later hardware revisions, much to the annoyance of we early adopters.
The iPhone's EDGE network capability has passable performance, and, with a little better caching, could feel snappier than it currently does. EDGE has the advantage of being widely deployed, and being efficient with battery power. But AT&T already has a much faster network, HSPDA, rolled out in many locations. My wife's phone, a Nokia N75, supports it, and it would be great if the iPhone had the option of supporting it. Supporting another network would add another component to the iPhone's jammed innards, possibly making it larger. It's 100% certain that future versions of the iPhone will support newer, faster networks. If the second generation iPhone supports HSPDA, many owners of 1G iPhones will be angry.
The lack of GPS capability in the iPhone is, in my opinion, its most serious flaw that's not fixable with a software update. The Google Maps feature is quite handy, and even supports traffic updates. When I was traveling from Rockville, MD to Annapolis a couple weeks ago, I was able to use Google Maps to see that the Capital Beltway was choked with standstill traffic. (though the only reason I was looking at Google Maps was because I was already stuck in standstill traffic. But it was still cool.) But as I was trying to navigate around Washington DC later on, I was constantly annoyed that, though I had the maps, I always had to spend several minutes first finding out where I was. Why oh why didn't Apple include a GPS chip, so the map would load up with my location in the middle? The next revision of the iPhone will almost certainly be equipped with GPS. There's just no good reason
So does this leave us out in the cold, with our non-GPS iPhones? That depends on Apple. There are two ways that our 1G iPhones could be modified to enable Google Maps to show us where we are. The first one is all software. If the iPhone is on the carrier's mobile phone network, triangulation between towers can provide a relatively accurate estimate of the device's location. It would be sufficient to make the Maps feature much more useful. It wouldn't be any good out in the wilderness, and wouldn't give you an exact location, or be able to calculate your speed or altitude like a real GPS can, but it would be a welcome feature that Apple could add if it wanted to.
The other way to provide GPS, which would be perfectly convenient in the car, would be to support an external GPS antenna over Bluetooth. In the car, this would actually be superior, because the antenna could be positioned for maximum signal strength. Less useful for walking around, though.
Apple could even provide an aftermarket built-in GPS capability if it wanted to. There's a company that makes a GSM SIM card with a GPS chip built-in. Apple, in coordination with the carrier) could sell (or give away) a replacement GPS-enabled SIM and a software update, and enable GPS use. This could be a revenue source, as there could be a one-time or monthly fee. I would definitely grumble if it were only available as a subscription, but I'd probably pay up to $50 one-time.
What Apple really needs to do, though, is not only enable GPS for maps, but integrate GPS location into many new and existing features: Localized business directories, localized movie listings, notification when you're nearby another iPhone user who's specified that they'd like to chat (including, perhaps, a dating-service-like profile that could revolutionize the singles bar scene), ability to location-stamp emails, blog postings, or microblogging services like Twitter or MySpace.