First thing that comes to mind for me is making sure you know if anyone uses Photodocs for any purpose. Photodocs' end of life is version 10.4, so it's not part of the version 11 client install. I've found that client version 10.2 has the most stable version of Photodocs, and have kept that on some servers and client machines.

If you use the Web Client internally without SSL, you won't be able to use Web Client 11 the same way - it requires SSL. Always good to get SSL set up anyway, but if you don't currently use it, make sure you have a plan to implement that with Web Client version 11. 

Lastly, I've only seen this within one customer environment, but I did an upgrade from 10.3 to 11 and started getting SSL errors with Windows client 10.3 when connecting to server 11. TLS/SSL between the server and client was not configured anyway, so was a pretty weird issue. Upgrading to client version 11 seemed to resolve it, so you may want to plan on needing to upgrade your Windows client instances fairly soon after upgrading your server.

That's all that I think I've run into.