Samsung phones have problems with GPS
the processor isn't faster
theres no keyboard
both OLED screens (u can argue for SUPER Oled if you want)
no revolutionary features
Nexus S = marketing hype
GPS - with the amount of work that Samsung has put into trying to fix the GPS on the Galaxy S phones, it appears that it is probably a hardware issue. I would venture to guess that Samsung will avoid that provider in the future.
Processor - depends on which one they put in there. If is the Orion processor with a dual core, it will be blazing. The thing you need to look at is the GPU. The G2 has a 800mhz processor, but has a very powerful GPU, which in turn makes it perform above and beyond what many 1ghz processors are capable of doing.
Keyboard - some argue for it, others argue against it. Personal preference.
Screen - while the sAMOLED screen of the Galaxy S is a thing of beauty, it does a number to the battery life. It is very easy to view in about any lighting condition though and is incredibly thin and light.
Revolutionary features - how about coming with stock Gingerbread? No mucking up the OS with a manufacturer's half-baked UI. How about the dual core processor (if it comes with one as rumored)? Point is, we do not know what it is going to come with and what new juicy tidbits that Google is going to throw at us.
Nexus S = a great opportunity.