Why would a program require a server operating system versus a workstation operating system?

The limit of 20 concurrent connections applies to SMB connections as well. You're really not thinking scalability if you limit yourself to a max of 20 clients.

The bigger issue is that you wouldn't be following your vendor's requirements. They don't support what you want to do, so don't do it. They likely don't QA it against non-server OSes, so you shouldn't run it on one.

Not to mention that using a workstation to perform a server's function makes systems admins cry.

Technically? This entirely depends on the program's architecture.

Sometimes there is a "real" reason; server OS's are tuned for server tasks rather than desktop tasks. They can be tuned for better network performance over delivering a better desktop experience.

Usually what it comes down to is the vendor doesn't want to support desktop issues. They have a set of systems they test on, and if you put it on something they didn't test, it will probably work just fine. BUT if you DO have trouble, they'll slap the pointer stick at a big poster that specifically states WE DON'T SUPPORT WHAT YOU DID and tell you you're not entitled to support, please stop clogging our tech support lines...

Last thing I think of offhand is licensing. For multiple users you usually need a server license for all your users; desktop workstations aren't licensed for this (depending on the OS.)

What it boils down to is that in most cases it only runs on servers because the vendor tells you to. Not a technical reason. Sometimes there is a real reason for the requirement. The only way to tell is to test it. Most sysadmins frown on this, though.

I believe the 20 concurrent connection limit is related to windows file sharing. So if it's not using windows filesharing and instead uses ip sockets directly then it should run fine. There may be other limits if the software utilises any services on the machine that are MS specific. But my guess is it'd technically run fine.

The other difference is that windows server tends to be tuned in a way that optimises it for serving things in the background, while desktop OS's like windows 7 are optimised to provide a better user experience.

As mentioned by the others though, the vendor may not want to support their product on anything other than windows server. You may find the installer prevents you from putting in on anything else. Try it and see how far you get.