Keep in mind that Windows Server 2008 R2 mainstream support ended on 1/15/2015. If you don't believe me, check out Microsoft's Server 2008 R2 lifecycle page for yourself:
All is not lost, however. Extended support--for 2008 R2 customers with valid service agreements--runs through 1/14/2020.
Still, as of this writing, that's only about four years away. And in case you're wondering, Server 2012 R2 mainstream support ends in 2018 (superseded by Server 2016), and extended support ends in 2023.
Others in my client base don't want to migrate to Server 2012 R2 because they dislike the GUI. I certainly can sympathize; it's just different enough from 2008 that the learning curve is steeper than I'd have liked. However, that's not a reason to not utilize a more up-to-date server operating system.
The "killer app" for getting 2012 R2 installed in my client sites, however, has proven to be DHCP High Availability.
With Server 2008 R2, there was no such thing as high availability for DHCP. You could set up two servers to hand out a split DHCP range, or set up servers in a failover cluster, but that was about it.
In Server 2012 R2, it's quite easy to set up two servers for DHCP high availability You simply set up DHCP on the primary server, then activate DHCP fault tolerance on the secondary server.
2012 R2 provides two modes DHCP fault tolerance: Hot Standby and Load Balance. The two modes are detailed in this Microsoft tech note:
Microsoft also has provided a great step-by-step guide to configuring DHCP failover:
In my network designs, since I usually have one physical DC (to provide network time sync and DNS in the event of a virtualization cluster malfunction) and one or more virtual DCs, it's maybe a 10-15 minute job to set up DHCP failover. Even in small business networks, I generally create a pair of 2012 R2 DCs (again, one physical and one virtual) and set them up for either Hot Standby or Load Balancing mode.