[DonaldPShimoda]

Off the top of my head: always-on display, additional sensors (compass, blood oxygen, ECG, more advanced accelerometer for fall detection), higher resolution display, different speaker system I think, slightly different form factor, uhhh... that's all I've got immediately but I think there's a bit more.

[r00fus]

Better processor. Wife has series 3 and I have a 4 - the 4 is 50% faster. I wouldn't buy a series 3 as it's just a bit too slow.

[DonaldPShimoda]

Oh absolutely right, I dunno how I missed that!

[suyash]

what about altimeter?

[DonaldPShimoda]

Another good point! Like I said, I was just going off of what I could remember offhand. :)