When I upgraded to the WM6.1 (RUU_Kaiser_CINGULAR_WWE_3.57.502.2_radio_sign_25. 88.40.05_1.65.21.18_Ship.exe), I found that my videos with file extension ".3gp" will no longer play.

I get a simple Windows Media error "Cannot play the file".

When I reverted back to the original WM6 the one that came with my AT&T Tilt (RUU_Kaiser_CINGULAR_WWE_1.62.502.0_radio_sign_22. 45.88.07_1.27.14.09R2_Ship.exe), my .3gp files would play normally.

Does anyone know why this is the case?

Thanks in advance.

They may have removed a codec from the player. I've only ever seen .3gp files from mobile phones, so if they shifted to a different format it's possible they dropped the other.

Hmmm... well this sucks... Right now, I'm not seeing any "value add" for my upgrading to WM6.1 The only thing I'm seeing differently is the threaded IM's, the zoom feature in IE, and the time "plug in" to the Today screen. Of these, the only real value to me is the Today screen. But since I use SPB mobile shell, it's somewhat redundant!

Am I missing any other feature?

If you're syncing OTA with an Exchange server, there were a number of improvements there, but those are in the background. Performance and memory handling were supposed to have been improved, but I didn't notice a difference there.

Personally, I'd dump Media Player for Pocket Viewer or the Core Player and see if they worked for you. It's a memory and performance hog unworthy of your time.