This post is mostly for my own information, but could be useful to other people while searching for the Virtual Machine Hardware features available within each release, starting with version 4 (ESX/ESXi 3.5) and ending with version 9 (ESXi 5.1).

I got this information off the VMware website (vSphere 5.1 documentation) and in my opinion the following features are the most important ones:

Version 4 (ESX/ESXi 3.5 and later)
64 GB RAM limit
4 Logical processors
4 Virtual NICs
128MB video memory

Version 7 (ESX/ESXi 4.x and later)
255 GB RAM limit
8 Logical processors
LSI Logic SAS and Paravirtual adapters
Virtual IDE disks
10 Virtual NICs
VMXNET3 NICs
128MB video memory
More guest operating systems supported

Version 8 (ESXi 5.0 and later)
1 TB RAM limit
32 Logical processors
LSI Logic SAS and Paravirtual adapters
Virtual IDE disks
10 Virtual NICs
VMXNET3 NICs
128MB video memory
SVGA 3D hardware acceleration
More guest operating systems supported

Version 9 (ESXi 5.1 and later)
1 TB RAM limit
64 Logical processors
LSI Logic SAS and Paravirtual adapters
Virtual IDE disks
10 Virtual NICs
VMXNET3 NICs
Nested hypervisor support
512MB video memory
SVGA 3D hardware acceleration
More guest operating systems supported