I own both a Nexus One and a Galaxy S. I will tell you the difference of the screens is that obvious. Yeah. Seriously.
But there are a few things that are off putting on the Galaxy S. Like the shameless iPhone mimicry. Like when you charge the phone, you get this big green battery indicator which looks exactly like the one on the iPhone. They put the screen wake on guess what, the middle button. Just like you know what. The TouchWiz application menu swipes left to right, like guess what, the iPhone, and no it does not scroll up and down like most Androids. There is also those dots that you see on the iPhone too.
I also notice a few apps that gives issues on the Galaxy S for some reason. No, they don't hang, but they seem to suddenly lag in a way I don't notice them do so in an HTC Android.
The body doesn't befit a high end smartphone too. It doesn't have the heft, smoothness and feel of a Nexus One or HTC Desire on the hand. It does feel tight and strong, looks like its not afraid to go take some licking.
But, collectively it has a lot of perks going beyond the bright and sharp screen:
Best factory lockscreen of any Android.
Best approach to a notification bar in any Android, like putting Wifi and Sound settings on the bar.
Comes with Swype keyboard. You need to enable it though.
Great software bundle, including Aldiko ereader
Mobile wifi hotspot - you don't have to wait for Froyo on this.
1.85gb to store apps, which is 10X more than the Nexus One (185mb). You can go apping like crazy here.
16gb built in, plus an extra SD card slot for more.
Samsung app store, for Samsung's own special Android apps.
Some great widgets that rival HTC's.
Generally, it all feels faster and smoother than a Nexus One. Except on some ficky apps.
An example of Samsung own Android app is the RoadSMS. Basically its an SMS text program that turns on your camera for a camera view, with the software keyboard superimposed. So you can text and keep a view of the road ahead.
Unlike HTC Sense, which decides to forego the Android 2.1 Gallery app, Samsung has the wisdom to keep this in, but with a twist --- they got Picasa sync. Any pics you got on Picasa will show up on Gallery. Only place where I see something similar is done is through the Sony Ericsson Mediascape for the Xperia X10.