Share your experience!
CoolDesign started one of these for the Mini, although I love my phone I thought it was a good idea so decided to start one for the Mini Pro (2011 version). Check out the other thread too (http://talk.sonyericsson.com/message/107847#107847) as I'm sure alot of features will be shared, I've copied some into this thread.
Please post enhancements only, not bugs or issues with your phone... unless of course it is something that needs enhancing! I'll try to copy them into this post to keep them all at the start.
Added (25/10/2011):

** Just to add uliwooly-wooly posted some useful apps which provide this functionality (the starred items). See here and here or scroll down.
Solved! Go to Solution.
preview sms you can install this app
https://market.android.com/details?id=de.lulusoft.locale.lockscreen&feature=search_result
backlight and data traffic
check the other POWER CONTROL widget I suggested above
camera sound
you need to flash a Global Generic firmware to your phone to have the option to turn off the sound
or use Quick Settings app -> volume control-> system volume
https://market.android.com/details?id=com.bwx.bequick&feature=search_result
volume keys to control the music player its a great idea but you can use the headphones to do that, press it once to stop/play the song, press it twice to skip a song, press it thrice and you'll go to the previous track
or you can use the app above and add this feature to the lockscreen
Now you will probably say, now come SE didn't included all this features, some of them make sense, but the others are features that aren't used by most users, like I don't want to have a notification message on my lock screen but it is there, and I wouldn't want to have a preview SMS on my lockscreen but some users do, the beauty of a Smartphone its not the phone itself is the market, with it's myriad of possibilities to customized your device
I agree with everything there... But would like to.... besides the "long press" headphones button to enable voice search, if the media player is running, it would go to the next song instead.
(In older phones like the Naite or Cedar, the physical buttons for the volume (on the side of the phone) worked as "Next" or "Back" in the media player too... would be GREAT to bring that feature back)
I believe you can do this already ... but you just do a quick double press... I'll try it again tonight but pretty sure it works.
Maybe double tap forward... long press back when music player is running?

Don't need this one anymore .... don't read me ... oh too late!
Yeah, I've tried everything, it doesn't work
... haha and yeah this "Forum" really needs an Edit button ..
Hi, good idea
I'd like to help an active participant in the 'cricket' forum: Login, then on the right side press 'Edit thread' - and you'll see the miracle 
lol ... why is it over there!! Thanks 
Hmm .. strange... I've just tried it, basically works like this for me:
(Locked or unlocked doesn't seem to matter)
If last player used was music player (the one that comes with the phone), one press plays the track then one press pauses the track. While playing, a double tap on the button on the headset skips to the next track.
If the last player used was the radio (again the included one), one press starts the radio and one press stops the radio. While playing, a double tap on the button skips to the next scanned station.
The volume keys do the volume... camera key does nothing.

at least your ambient light sensor is working, with the Arc (the flagship phone ) it's not working and it hasn't worked since it was released, not even after all the updates, what a shame
photo/video compression we are all experiencing this (seems to be an xperia issue, at least with the new generation 2011)
fade in alarm with ringtones can be done with apps
like this one
https://market.android.com/details?id=com.mix1009.ringtoneat&feature=search_result
lockscreen widget to control music it's also available on the market
apps like this https://market.android.com/details?id=com.teslacoilsw.widgetlocker&feature=search_result
3 settings for brightness
use this widget
https://market.android.com/details?id=com.siriusapplications.eclairwidgets&feature=search_result
landscape homescreen, I agree that it should be included not only to slide out phones but to ALL Xperias
preview sms you can install this app
https://market.android.com/details?id=de.lulusoft.locale.lockscreen&feature=search_result
backlight and data traffic
check the other POWER CONTROL widget I suggested above
camera sound
you need to flash a Global Generic firmware to your phone to have the option to turn off the sound
or use Quick Settings app -> volume control-> system volume
https://market.android.com/details?id=com.bwx.bequick&feature=search_result
volume keys to control the music player its a great idea but you can use the headphones to do that, press it once to stop/play the song, press it twice to skip a song, press it thrice and you'll go to the previous track
or you can use the app above and add this feature to the lockscreen
Now you will probably say, now come SE didn't included all this features, some of them make sense, but the others are features that aren't used by most users, like I don't want to have a notification message on my lock screen but it is there, and I wouldn't want to have a preview SMS on my lockscreen but some users do, the beauty of a Smartphone its not the phone itself is the market, with it's myriad of possibilities to customized your device