The biggest thing that makes a difference for me is the simple existence of file browsers. With a browser like ES File Explorer, I can e.g. copy music, movies etc. from my NAS across wifi without having to plug my device in. I can zip folders up and unzip them, and applications don't act as silos for their specific data, and nor does data need to be shoe-horned into flat, generic categories - music, photos, movies.
The file browser makes all the difference in perception. It feels like the difference between a classic iPod and a PC. One is a toy, the other a tool.
The combination of notifications and multitasking: for example, when I get an email, I can pull down the bar and see its title. If I need to deal with it, I can tap on it and reply, then press the back button and I'm sent back to where I came from, rather than having to go to a home screen, etc.
And I really like the widgets. The Google calendar widget shows my upcoming appointments, and it's on the center home screen. The AutoRotate OnOff widget hugely improves usability if you're e.g. in bed: you can disable auto-rotation in web pages. Actually, I think auto-rotation should be disabled by default, as it's almost never useful - always more of a gimmick than a feature. But you have to jailbreak the iPhone to disable auto-rotation.
The navigation, tethering etc. are all just gravy. I've had tethering for a long time on my ancient K800i, both over USB and bluetooth, and it's always handy for airports and hotels abroad.
That's another thing: cost. I use a UK O2 pay as you go sim, and spend about 11 GBP per month - 8 of that, IIRC, is for internet usage. But when I'm abroad, I use my Irish Vodafone sim and relegate the O2 to my K800i. The Vodafone sim works in the US and all across Europe, and costs me 0.99 EUR/day for 50MB/day. That's enough to cover my casual daily Google Maps and browsing usage when I'm travelling, so I'm never without internet access wherever I am, whether it's Albania or California.