The main reasons you'll want to pursue an enterprise-level virtualization solution are mindshare, support, manageability, and feature-set.
Mindshare is important because virtualization is an investment in a technology, an investment that requires platform longevity. Nobody wants to be the one who picked the wrong tech solution. So the major players in the space (VMware, Microsoft, Citrix, KVM) all have some momentum behind them. This affects third-party applications and plugins; think of SAN-integration or backup software. More mature virtualization suites have APIs that are leveraged by other products. It's natural that more solutions would be developed for more popular platforms.
Support is linked to mindshare. I'm constantly battling bugs and obscure problems with my Citrix Xenserver/Cloudstack solution. Due to mindshare and general knowledge of the solution being an order of magnitude smaller than something like Hyper-V or VMware, I have to rely heavily on Citrix support, bugfixes and trial-and-error to fix problems. Other solutions would have more community forums and of course, more people who've vetted the technology.
Manageability and feature-set are key as well. Hypervisors today all provide similar raw capabilities: the ability to host multiple guest virtual machines and different operating systems on physical hardware nodes. It's how well they're packaged together and can be managed that shapes perception of the overall solution. Automation, monitoring, reporting, an ability to troubleshoot performance issues, and ease of installation are some important attributes. Also, any enterprise solution will have some ability to migrate virtual machine guests live between hosts and/or storage.