One feature that is really missing at Citrix XenServer compared to VMware vSphere is OS Customization. My dear co-blogger Birk (the VMware guy) will never stop to underline this. At vSphere settings like Hostname, Network configuration, Domain membership, Licensing can be defined in specifications and applied to templates and VM images.
Recently I had to troubleshoot network performance issues for VMs running on Citrix XenServer 6.0.2. All VMs were running W2k8R2 and it seems that CIFS copy jobs where incredible slow. The network itself couldn’t be the problem as all VMs were connected to the same virtual network on only one XenServer.