Generally a lot more stuff than the average AVR, so (IIRC) you have a multi level interrupt controller, a DMA unit, more adaptable clock / power configuration, and just more of things like UARTS and ADCs (which tended to have more features) and so on[1]. It feels more like an AVR core bolted onto something nearer to the “everything and a kitchen sink” peripheral set you get with a 32 bit ARM MCU. The projects I worked on weren’t particularly super cost sensitive though, so I’m perhaps a bit isolated from the “every cent matters” level.

(Also, you’d probably just use one of those 32 bit ARM chips nowadays).

[1] I don’t have an encyclopaedic knowledge of the AVR range and haven’t looked at in a couple of years, so I’m sure some of the advanced stuff is available on Mega models.