The largest barrier to getting anything in the workplace but windows is a common ground with which you can collaborate and work. If you want to replace say, sharepoint, you can expect to have to sell everyone on the idea. your replacement needs to work seamlessly, just like sharepoint.
if you have a vmware deployment, linux is pretty much a non-starter as anything but a guest OS. you cant administer vsphere from linux, at least not in a way that wont make you hate your life. Many timecard systems and in house software packages might be predicated entirely on windows Internet Explorer, so the loss of ADP might piss off accounting. determine your userbase and its needs first.
Switching people from exchange is a daunting task, but egroupware and others can step up to the plate with a web-based UI. its also a huge cost saver. Whether or not your office wants that is another matter entirely. your linux systems will have to authenticate to AD, and never the other way around because windows just cant. while Libreoffice sure is a nice replacement for a new office, its a disaster when it comes to some of the finer points of complex excel spreadsheets, pivot tables, and the latest doc format. Lync, er, microsoft communicator as it was once called, has tentative support in linux but you lose helpful features like auto away and auto populate and that "call this person" feature I wonder if anyone ever uses..
doing this isnt easy. Ive spent 5 years of my career doing it, and the biggest hurdle is going to be your users. They want features like desktop sharing for meetings and gantt charts for planning. Linux doesnt really 'get' it like microsoft. The key is to make sure the channels of communication between windows users and linux users, be they desktop application level or enterprise, is uninterrupted. sometimes a quick switch from say lync to jabber is best. in other places you might want to phase things like sharepoint out over time. make sure you know how they work, and have a plan to provide a service that helps them achieve what theyre being paid to do.
another pitfall to be wary of is Microsofts jagged edge. Decreasing site licenses will beget unforseen costs like losing your Azure discount or more expensive license seats overall. the purchase terms will also change randomly and rapidly in an attempt to kill your linux idea from the management down (they do this to force meetings with your managers, who in turn dont invite you because its about a budget and not a computer to them.) Once I weaned a prior company off lync and exchange, I had microsoft representatives drop in entirely unannounced and ask for a meeting with almost every manager they could find (and me.) They will hound you with phonecalls, bombard you with junk mail, and chew up your time like never before. They do not like being shown the door.