Historically, one of my biggest little annoyances with a VMware version upgrade has been upgrading VMware tools. In vSphere 5.5, VMware added the ability to update tools without a reboot. If you were manually kicking off the tools upgrade all you need to do is enter the following features into the advanced settings box /s /v"/qn REBOOT=ReallySuppress? This is great and causes the tool update to only lose one ping instead of requiring a full restart.
After performing an upgrade from ESXi 4.1/ESXi 5.0 to ESXi 5.5 u2 I noticed increased latency events on hosts. More troubling, the affected hosts were frequently dropping all presented datastores, though they would reconnect within a few seconds. The events may appear in your event log as below: While there are many possible causes to explore these sorts of connectivity issues, one that is often overlooked is how ESXi heartbeating to the datastores.