Currently I use the Nokia N82. Although I don't change my phone as often as I used to, I still get the occasional impulse to try something different. I bought a Sony Ericsson W890i just the other day, using an upgrade I had available, but it only lasted two days - the speaker is just awful! Ringtones and text alerts sound distorted.
So, for the next phone I get this is going to be what I look for:
3G - As a minimum, the phone must be 3G
Bluetooth - Almost a default feature these days, but I expect my phone to be Bluetooth 2.0 compatible, for stereo connectivity
Form factor - I like candybar phones, I do not like slide, nor clamshell.
Keypad - My phone has to have a usable keypad. Messaging on my phone is by far the biggest usage, text, email and IM.
Battery - My phone *must* have a decent battery. All the features in the world mean nothing if the phone cannot survive at least one day of solid usage as an absolute minimum.
Size - I don't want a bulky phone. Not too long, and not too thick.
Java support - My phone has to have good java support, to at least run the java GMail app and Opera Mini. Opera Mini is especially necessary if my phone has a poor native browser.
Email support - I wasn't sure if this would be an required option or not, as I use GMail, I mainly login via a bookmark on my phone's browser. But for sending email direct from the phone, it is easier to compose in the phone's email client, so decent email support gets on the must have list.
Alarm Clock - The most used feature on all my phones. If I can manually adjust the snooze time, so much the better.
Optional features on my phone are:
HSDPA - Ideally, the phone I buy should support HSDPA
WiFi - This is an optional extra. I rarely use WiFi, but it's a nice feature to have if possible
Operating system - S60 is by far the best I've ever used on any phone, and it has just got better and better over the years. But I'm not so blind to S60 as to not consider other phones. I like Nokia S40, UIQ, and standard SE phones too. But if a phone carries S60 it is a definite plus
Customisability - What I mean is I want control over the shortcuts on my phone, on how the display looks, on what appears on the standby screen and what doesn't.
MP3 Player - I like to listen to music on my phone, but it isn't essential.
Radio - As above. A radio is a useful option to have, but not essential.
Camera - So far down the list. I really am not bothered about a camera on my phone. If there is one, I'd like it to be worthwhile, to take decent pics, but I am not fussed too much.
GPS - Including GPS support is a nice touch, but overall I think I've used Nokia Maps three times in the last six months, and two of those occasions was because I could, not because I needed to. An application like Sports Tracker is useful, but not enough to make GPS vital.
Profiles - I like to be able to adjust the tone settings on my phone to my own comfort, that means being able to set different ring tones and text alerts for each separate profile.
I think I've covered the essentials and desirables. There's nothing extreme in my list, I think my requirements are quite conservative really. My N82 covers all of this (apart from maybe a decent keypad and size), I would want my next phone to cover all the essentials and as many desirables as possible.