SharePoint
I've been working recently on a SharePoint Portal Server 2003 implementation. Frankly, it's frustrating. Even though the product is officially "released" (at least to Partners, who are now implementing it), there is absolutely no useful documentation on it.

I'm trying to accomplish some fairly simple functionality, and I am just spinning my wheels. The SDK is incomplete. The object model is documented, as far as what objects exist, there is no useful information on how to use the OM. I suppose this is what it's like to be one of the first to do real projects using a brand new product.