For instance the RealPlayer plays MP3s just fine, and runs in the background, but it has no concept of the Nav buttons (which is especially annoying for reasons I will explain below).
The camera works great, even in very low light, and the videos are cool too (though others have mentioned the desire to be able to adjust the compression settings), but I'm always switching between picture and movie and every time I switch it points back at the built in memory instead of the SD card. Now since the builtin memory is 19MB and my SD card is 1GB, its pretty obvious if somewhat annoying, but even still I end up with pictures and movies on the built-in space. And of course there's no command to move the picture (though there is a copy command which then you have to shuffle around and delete the source).
The browser shows an amazing amount of information using the small font, but it reveals the throwback nature of the OS all to often, doing its processing and not letting the application or even the entire machine to respond to other events or actions. And I soo miss tabs. Note to Palm: could we add the option to not load images (clickable option on screen, or site by site preference?) Just pulling one page from Ebay ends up using > 500k, and Yahoo mail is perfectly readable up until the moment it loads its screen formatting graphics.
You can also sense how kludged the original vision for the PalmOS has gotten over time by keeping an eye on which apps show any status information about the device like battery and signal strength. It would be really helpful to know I'm loosing my signal while in the middle of a SSH session or while chatting on IM, but they're blank about anything beyond their own function. Not that I blame the app. Palm needs to create something like the Mac titlebar and let the device or system utilities put their icons up there so there's consistant information when moving between apps.
Of course the pinnacle of my user experience rollercoaster so far has been the fact that my Palm is defective. Broken. Going back to the shop after only two days. For some reason the touchpad won't stay lined up. I do an alignment, tap a few spots, and Bam! its off by more than a line vertically. Doesn't seem to be a widespread problem, but I've seen signs of a few other people doing exchanges for bad hardware.
I'm still happy with the Treo overall, its just now I don't have to worry about the fingerprints the kids are putting all over the screen on this unit as I'll have another as soon as the exchange unit shows up.