I remember endless stories of troubles with Sharepoint. I think it's very buggy by design (one example: it stores it's users on multiple places, not in 1 db). You need enormous quantities of hardware and a large amount of money on licencing to get it running. Microsoft is making lots of money on support I guess, especially since the release of this product.
It works not well with non-microsoft software (like browers). When I was using it it had no support for non-microsoft formats, maybe this has changed.
I think it's more proprietary than Alfresco which is released as open source.