For one the plastic casing was as cheap as you can get. I dropped my phone once while getting out of my car less then a foot off the ground and the thing looked like I threw it against the wall.

Second the gps did not work, this was not an uncommon issue, so uncommon that T-Mobile replaced my phone no questions asked for a G2.

Third the software they put on the phone, while not an issue for this phone, was slow, buggy and hard crashed my phone at least once a week. This coming from the G1, running community build roms, which hard crashed maybe twice in the 2 years I owned it.

Here's a fix for the GPS problem:

http://cellphonesignal.net/google-nexus-s-a-gps-system-leaks...

Thats interesting the Nexus S has the same issue. And while this fix put a band-aid on the problem it in no way fixed the issue at all. And sometimes it actually made it worse.

Samsung was also notoriously slow in releasing the Froyo update for their phones

Correction: IS slow in releasing Froyo. Still don't have it for my Mesmerize.