It's been a while, but here are the issues I remember:
* GPS was completely broken (a known and admitted issue with the T-Mobile variant of the Galaxy). If you disabled it, you could use tower triangulation to get a rough position, but that made it unusable for things like driving directions.
* Battery life was terrible (probably not specific to that model, but it didn't get through a day the whole time I had it).
* The camera was awful (not unusual for smartphones at the time, but my old crappy N73 had a camera that was easily 10x better).
* If you unplugged the headset, the phone would automatically launch the music player and start playing music through the speaker, even if the music player wasn't running.
* USB mass storage mode didn't work, so you couldn't move files to it. The only workaround was to turn on debugging on the phone, which would then let it show up in as a USB device, but you'd have to enable/disable debug every time you plugged it in, as leaving it in debug made the screen not turn off.
* Lots of crap applications that you couldn't remove.
* Most of the menus had misspellings or bizarre word choices, like it had been passed through Google Translate (a nitpick concern, I realize, but it didn't give me a lot of faith in the quality control of Samsung).
Then there were all the issues with Android itself, which aren't specific to the phone, and I really don't want to hash out here, as some of them have been fixed by now, and I really have no desire to get into a debate about the merits of the platform.
I spent the next six months or so after returning the phone to see if the issues ever got addressed, just to see what would have happened if I kept it, and it seemed like most of them were still known issues. That made me feel better about my decision to return the phone.
I can identify with most of these issues on my Captivate, and that many of them were improved or resolved by installing Cyanogenmod (the big issues, GPS and camera, remain). That isn't a legitimate option for the vast majority or smartphone buyers, but it has extended my patience with an expensive purchase not living up to the claims of its maker.
* GPS was completely broken (a known and admitted issue with the T-Mobile variant of the Galaxy). If you disabled it, you could use tower triangulation to get a rough position, but that made it unusable for things like driving directions.
* Battery life was terrible (probably not specific to that model, but it didn't get through a day the whole time I had it).
* The camera was awful (not unusual for smartphones at the time, but my old crappy N73 had a camera that was easily 10x better).
* If you unplugged the headset, the phone would automatically launch the music player and start playing music through the speaker, even if the music player wasn't running.
* USB mass storage mode didn't work, so you couldn't move files to it. The only workaround was to turn on debugging on the phone, which would then let it show up in as a USB device, but you'd have to enable/disable debug every time you plugged it in, as leaving it in debug made the screen not turn off.
* Lots of crap applications that you couldn't remove.
* Most of the menus had misspellings or bizarre word choices, like it had been passed through Google Translate (a nitpick concern, I realize, but it didn't give me a lot of faith in the quality control of Samsung).
Then there were all the issues with Android itself, which aren't specific to the phone, and I really don't want to hash out here, as some of them have been fixed by now, and I really have no desire to get into a debate about the merits of the platform.
I spent the next six months or so after returning the phone to see if the issues ever got addressed, just to see what would have happened if I kept it, and it seemed like most of them were still known issues. That made me feel better about my decision to return the phone.