The upgrade to Nook's Cranny has many fancy new features, including:
A wider selection of items for purchase
More tools (sporting new designs like Star Nets and Elephant Watering Cans - which are, alas, just as breakable as the boring ones you'd craft yourself)
More furnishings to purchase (some on the extremely expensive end)
Expanded wallpaper / flooring selections every day
A timer! for... timing things, and... stuff
Two "hot items" per day (versus just one at the previous shop) so you can make bank
However, it appears from initial research that these "hot items" in the expanded shop are also much more difficult to find or craft