The 4040 added some instructions to the 4004 and was a bit better. I seem to remember something about the stack, but it's been so long... Early BYTE magazine actually had articles with 8008 assembler I remember. The early flavors of micros were diverse. Feature rich machines were damn expensive in general. People would cobble together whatever they could, instruction sets and features of processors were actively debated. The major camps seemed to be:
-- RCA COSMAC 1802. You could build a minimal system cheaply. Speed wise this was fairly slow, but it was a real processor. Lots of registers. Architecturally it wasn't as bad as some people like to say. Yes, you could easily implement a CALL/JSR and RTS too.
-- Nat Semi PACE/IMP-16. Most of the folks who went this route seemed to want more 'mini' style features - e.g. 16 bit instruction sets. These were very slow performance wise. Minis based around them were cheaper than the big-name mini's but seemed super slow. Met people who said their early 8080 micro kicked the mini's butt in terms of performance.
-- Single & multi chip direct from mini architectures. Stuff like TI 990, DG NOVA DEC PDP 8 and PDP-11. The TMS9900 was a notable contender.
-- Motorola 6800 and Intel 8080 - Both were popular, and much faster than stuff like the PACE / IMP-16 / 1802. They were usually simpler to design against and often faster than the single chip mini's derived stuff too (excluding maybe the TMS 9900 ).
-- MOS Technology 6502 at the same clock speed was usually equal or faster than a 6800 / 8080 for most stuff. It was super cheap too.
-- Zilog Z-80 A notably enhanced 8080. Loads more instructions. Very non orthogonal. Easy to interface to DRAM. Very very popular. The Z-80 pretty much gobbled up the 8080 family. Even when the 8085 came out, people went for Z80's. CP/M & S100 bus stuff, but lots of other stuff used Z80's like home micros etc.
The 6502 also gobbled up a bunch of designs. Motorola responded with the 6809 which is a really nice 8 bit to program. But by then the world was moving on to 16/32 bit stuff.
One of the most interesting things (to me) is that the Mini folks totally screwed up and ended up being wiped out. One wonders what would have happened if DG or DEC had produced a decent microprocessor based on the NOVA or PDP-11 at a low cost. DEC sort of did this with the LSI-11, but there was always this tension of the high-margin mini stuff being eaten from the bottom up. What if they'd been smart enough to pragmatically say - our high margin business is going to get destroyed by microprocessors, do we want them to be our microprocessors or someone elses?
One thing people forget about Intel is they did a superb job of supporting their stuff / had great sample designs that made getting started easier. Motorola for the 68000 actually actively discouraged folks and banned their engineers from talking to people who wanted to use the 68000 in the early days. Read D-TACK GROUNDED for details. It seems absurd right?
The general thing seems to have been that everyone wanted the new micros to go into the nice old high-margin mini business. Except they didn't. Cheap powerful MPU's meant people didn't want to pay 10x the MPU cost for software licensing fees or other garbage. They figured out how to cheaply hook video up to the microprocessors and keyboards. They figure out how to cheaply hook up mass storage. They built what was needed, and what worked got used. Not always entirely fair - there are plenty of amazing designs that never got traction, but life ain't fair.