Although it's good for trying to provide some info, please be more accurate...
The K850i is right now the flagship Cyber-Shot camera phone for Sony Ericsson. Although the phone does have some bugs, when it works well, it is an excellent performer. A few of the people I know that don't have any problems like their K850i.
Also, I don't see how the "features" are not as good. It runs the PSP menu UI over the standard on the mentioned models you stated, which are below.
I have owned each of these phones, so I do have some experience with them. Let's do a comparison...
K850i: Flagship 5MP Cyber-Shot camera phone with AutoFocus, PSP menu grid, touch sensitive buttons, US 3G support for faster streaming and downloading over EDGE that all those other listed phones only have, 3 LED photo light AND Xenon flash for picture and video taking, 30fps for smoothe video recording, MicroSD AND M2 memory card support, A2DP stereo bluetooth, and basically what those other phones listed can do.
W580i: 2MP camera with NO AutoFocus and NO flash, Walkman 2.0 app, Pedometer app, M2 Memory Stick Micro support only for memry cards, A2DP, and the usual stuff included on high to mid tier Sony Ericsson phones.
S500: Virtually the same phone as the W580i with the exception of the Pedometer app. If people are curious, yes you can flash the the phone's firmware from a S500i to that of a W580i if you prefer the more subtle look of the S500i over the W580i. The only thing that won't be working is the Pedometer because of the hardware.
W810i: 2MP camera with AF and dual LED flash, older Walkman 1.0 app, only Memory Stick Pro Duo memory card support, NO a2DP stereo bluetooth, the usual as well.
K800/K790: 3.2MP camera with AF, Xenon flash only, clunky Media Player, A2DP stereo bluetooth, only M2 Memory Stick Micro memory card support, sometimes the joystick can wear down with extended use, the usual stuff as well.
iPhone: Other than the sleek web browser and touch screen interface, there's not much said for the camera. It's a 2MP camera with no AF...and in terms of features, it doesn't give a whole lot for effects when taking pictures. Browsing the web with Wi-Fi is excellent. But when it comes to EDGE speeds, it is slow.
Although there are some that think that having more MP means you're going to get a better camera, that isn't the case as long as you have excellent hardware for optics. Some people are content when staying with a 3MP camera because they have excellent optics. But if you are set with staying with camera specs that are similar to the K850i, check out the link below for comparable phones.
Link: 5MP camera phones
If you want the best out of those options, go with the Nokia N82.