Having upgraded from the 5500 Sport to an E51, I thought it might help to give a summary of the improvements in the traditional form of 10 items
In no particular order:
1) More memory and faster processor. You can run more apps, they start up quicker, and you can install more on the phone rather than the memory card.
2) Better keyboard (quieter, too. I think I can get away with using this one in bed. The key-click on the 5500 was too loud.
) The only problem I've had with the keyboard so far is clicking on the 'delete' key when paging down with the D pad.
3) The camera lens is better positioned. On the 5500, I find my fingers close to the lens when holding the camera with my thumb on the 'Ok' button. On the E51, partly due to the slightly greater length of the phone, the lens is clear of my fingers.
4) Extra, programmable, keys. Now you can, for example, start Handy Calendar with one key press.
5) Bigger screen (both in area and resolution). I managed with the 208x208 of the 5500 for a while, but I think QVGA is really the minimum usable resolution for presenting information. You get 12 items in the menu instead of 9, and more content in general.
6) Better support for video. I can watch video podcasts now (and record in QVGA too).
7) WiFi. This means I don't have to worry about my monthly data limit, as I can do the bulk of downloads at home for free.
8) The E51 will read emails aloud, as well as SMS messages (the 5500 only read SMS).
9) 2.5mm jack for the headphones is much easier to use than the pop-port which I found very fiddly (partly because of the waterproof cover on the 5500, of course).
10) Generally improved interface. Not sure if that is specifically FP1, but things like the neater speed dial setup, collecting all the application settings under the main Settings app, the setup wizards on the active standy by default, and so on.
Torch; it was very useful on occasion to have the LED torch of the 5500. This is probably the main thing I'll miss.
Accelerometer; the one in the 5500 wasn't made much use of, but in the E51 it could have been used to detect screen orientation, etc.
Fitness tracker; the pedometer based on the accelerometer was a nice idea, but in practice I didn't find it very accurate, especially for cycling.
The tap controls for the music player might have been more useful if I'd visited the gym as much as I should have done.
Edit (pencil) key; not present on the E51. Supposedly its function is performed by the # key, but it doesn't always seem to be the case.
Ruggedisation; the E51 is probably not as wear- and water-resistant as the 5500 (and the 5140i before it). On the other hand, I suspect (and hope) that it will be as tough as the 6310i, its spiritual predecessor, which survived some rough handling over the years, though I shall be treating the E51 with a lot more respect.