probably the charging speed and also the screen size. for an old man like me, screen size is much appreciated 

Definitely faster charging on my series 7, I still have my series 6. So far my 1 week old series 7 stainless steel have no visible scratches yet, my series 6 is about 5 months old already have scuffs but not that noticeable. I don’t use case or glass protector on both,

It makes them too bulky

I came from an ancient but trustworthy aluminium 42mm S2. When i 1st got my SS silver 45mm S7 i was amazed by how far apple watch has evolved.

Everything about it is new (to me). 42mm -> 45mm is a huge noticeable diff. Always on display. Fast charging.

its larger screen is a bigger deal than i’d imagine. The s7 uses its bigger screen to great effect, giving a genuine upgrade over older models.