Is there any technical reason why I shouldn't use windows server 2012?

My company is in the process of rolling out Active Directory enterprise wide, with office 365 syncronization, Lync, Exchange, and Outlook.

We currently have no AD (yet); there are 400+ users in 5 separate offices.

The quandary we are facing is whether we should initially deploy Windows Server 2012, or 2008R2.

Certain elements are afraid of unknown issues with 2012, and one of our contractors suggested that we use 2008 because it has a larger knowledge base.

I've done all my testing in 2012, and I know that either one will work for our purposes. It's a fairly simple deployment (2 DCs and 1 ADFS server).

We will also be adding WDS, WSUS, Sharepoint, and WebHelpDesk.

Is there any valid technical reason that we should not go with the latest available version, assuming this is a complete description of the environment, or any caveat with Windows Server 2012 that we should be aware of?

The only reason I can suggest for delaying a move to 2012, is that you have an existing domain architecture that won't cleanly upgrade. But since you have NO Active directory deploy yet, I take it that you're using the whole LAN in "workgroup" mode and without any domain at all, this is a trivially easy case to install Server 2012 on, and the new tools are the easiest tools yet, for creating a brand new ActiveDirectory Domain Controller and related infrastructure. If however, you mean you're still using Windows NT Server, pre-Active-Directory-domains, then I don't know if you're even going to get that all the way up to 2008 R2 in one go. You might want to update your question and specify what you mean by "having No AD" right now.

Get a book, either way you go, and learn about ActiveDirectory before you deploy it. I am running a mixed 2012/2008R2 domain server environment at our office, and I find that while I know the existing 2008R2 tools better than the 2012 tools, the 2012 tools are superior in every way, except for certain odd little tasks, and you can still access the old 2008R2 user interfaces if you want to, in 2012.

Both Windows Server 2008 R2 and 2012 are fantastic products. I see no reason not to deploy the latest version (Server 2012) in small LAN environments or for brand new ActiveDirectory deployments, and many technically advanced features in 2012, that are good reasons to prefer deploying 2012.

In large network environments with existing 2008 R2 infrastructure, obviously, there would be sensible reasons to deploy more 2008 R2 domain controllers, instead of moving only part of a huge 2008R2 network's servers up to 2012.

The General Case: use $LATEST_VERSION unless you have a specific need not to.

You almost certainly fall into the general case, even more so since you lack "legacy" anything to hold you back.

It's true there could be unknown "issues" with 2012; but considering 2012 is mostly an incremental change to 2008R2, and the relatively extensive testing already done, I'm confident in considering it production ready (along with a large number of other users). You'll find a few talking-head websites that argue the days for waiting for SP1 before considering a Windows OS Production Ready are dead, and have been for sometime.

Speaking from the perspective of someone who has several times inherited "safe" versions I urge you to use the latest unless there is a real and unavoidable reason not to. While to some it might seem safer to use an old version on the basis that there should be no surprises, that's exactly what testing is for.

Quite simply, without a real reason to do otherwise, it makes no sense to install a version that is already 4 or 5 years old. Sure it might appear to make a little sense today but what about a few months from now, or a year or 2 down the track. Why create the extra work of upgrading later when there's no need to do so?

I would generally have said the latest version, but....

IMHO let other people Guinea pigs for the initial release! it's so radically different that there are bound to be some nasty surprises waiting to be ironed out in SP1. Wait for that and then you can consider an upgrade. Unless of course there is some needed feature in 2012 that you need right now.