It is not uncommon for microprocessor system designers to copy elements of design from other microprocessor systems; the Z80, extensively copying design elements of the 8080, is a notable example.
That the MOS 65xx series copied design elements from the Motorola 68xx series is not surprising; many of the designers working on that series had actually come from Motorola, leaving because Motorola was not open to their idea of doing a new CPU that could be manufactured considerably more cheaply than the 6800. One obvious direct copy was the MOS 6520 PIA, which is substantially the same as a Motorola 6821.
What elements of the Motorola 6800 that were substantially characteristic to it, rather than being "a common way of doing things," were copied in the design of the MOS 6502?
By "a common way of doing things" I'm talking about design elements that were widespread amongst other processors (microprocessors or otherwise), versus design elements that that you can clearly identify as being those of the 6800.
An example of the former would be, "an 8-bit wide accumulator named A on which you perform ALU operations that set flags," which describes not only the 6800 and 6502, but also the Intel 8080.
An example of the latter might be, "an additional addressing mode on many instructions with a shorter address within a subset of memory" (i.e., the "zero page" addressing mode), though I am not myself sure if that didn't exist on other processors before the 6800.
