After playing around with the phone for a bit i've come across some points that could really be improved in the media player.

Aside from the obvious, like better codec support, here are some of the things i've found:

-No folder view for music playing: Say you have a large directory of MP3's cobbled together from various CD's, there's no way to just start playing that directory.
The Music player has a couple of views like albums but if you have a large pile of various MP3's they just clog up all the possible views.
The only way around would be to make playlists, and that has it's own problems.
A folder view would really help out.

-UPNP support is very twitchy: After a fresh reboot, my upnp server (thecus NAS) mostly shows up fine, but it can dissapear without notice. If it does the only way to get it back it to give the phone a reboot.
It keeps on showing up fine on the other devices, but not the phone.
Have tried 4 different UPNP servers like gmediaserver, mediatomb, built in upnp server on thecus, and ushare on different machines but it's the same on all of them. They show up fine and then all dissapear without any particular reason.
Having some control would be nice, like a "scan" button in the context menu. (if anyone knows a workaround, that'd be great)
Ofcourse if it just keeps showing up that'd be even better

-No scolling in movies played from UPNP: Pretty much what it says on the box. Watching a 3 hour movie without the ability to restart in the middle for whatever reason is a real pain.

-Black bars aren't black: On playing most DIVX or XVID files (the ones that do play ofcourse) the black bars on top and bottom aren't actually black, they're dark grey. Switching from the media player to something else and back (CTRL + Backspace) fixes it.
Looks like the UI isn't redrawn correctly.

-After adding new media it doesn't refresh properly: Happened a couple of times now. You add a couple of movies, or some music, and you disconnect and start up the media player to find out the stuff you added is not listed. Shutting down the app and starting it back up usually starts it refreshing.
Would be nice if this were more consistent.

-No keyscrolling in lists: Would be nice. I like to scroll with the up and down arrow while looking for a file to play, but they don't work.

That's everything that bothers me (so far ).
I'll try and keep this updated with what i find, and maybe add a couple of bugreports when i find/make 'em.