2) First true multi-tasking operating system that came standard on a PC (Unix machines weren't PCs back then they were workstations)
Xenix (Microsoft Unix - yes, really!) was on the PC in 1983, it almost certainly had pre-emptive multitasking.
3) First computer to use co-processor chips that function independent of the CPU (i.e. the Bimmer (or super blitter).
Both the C64 and Atari 400/800 had custom chips.
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The Amiga didn't invent everything it did, much was done before. What the Amiga did was to put it all together in one place for the first time:
Fast 32 bit CPU
Multitasking OS
Drivers - individual programs used to support different printers (etc) directly.
GUI
High colour acelerated graphics
Sampled audio
