Full disclosure: I am an Alfresco SI partner. Alfresco is actually a SharePoint server and it has an open source version and a fully supported version. If you like what SharePoint does but don't like MS, give Alfresco a try. In addition Alfresco supports the draft CMIS which should allow it to interoperate with other DMS should you choose to swap out the back end.

Others have talked about Documentum and other open source systems, I spent 6 months looking at systems like Nuxio, Hippo, basically all the big open source players. Alfresco is by far the best supported and easiest to customize. Out of the box, the interface isn't pretty, but it is usable. They have a collaboration tool and a traditional ECM interface (both browser based). If you are looking at closed source systems, Alfresco beats them all on price hands down. Documentum can cost 7 figures with customization and it is very difficult to use. There is a whitepaper here on total cost of ownership: http://www.alfresco.com/products/whitepapers/.

Oh and Alfresco is extremley scalable. SharePoint does not do delta's with versioning, so you change some metadata on an 80 MB PDF file and you now are using 160 MB. SharePoint becomes pretty unusable before you hit 100K docs in the repository. Alfresco can handle 100 million docs and still be very usable.

Alfresco does everything you require. Why are you looking into other solutions? With Alfresco you can keep track of comments on documents or have a conversation with others regarding the document which is archived along with the document. Alfresco Share allows you to view documents with a flash front end so that you never have to download the documents into Word, Excel or Adobe Reader.