Additional languages, the ability to send two pictures or video via MMS, enhanced performance to support 256MB RAM devices... I think that's about it (that's been confirmed).
Rumoured: folder support & customisation of live tiles.
Yeah, I wouldn't really call it an 'update' as such. It looks to be more about opening the platform up to lower spec devices. It would be nice if there is some goodies thrown in, but I can't see it with Apollo being released a few months after
Apollo is due this autumn. In the worst case scenario, and we don't update, my guess is that WP7.5 will be faded out to Tango devices only. Just my thoughts.