It's not limited to DVCS or open source. Commercial tools such as ClearCase, AllFusion/Harvest, and Dimensions still handle large binary assets poorly.
I don't think you'll find a satisfactory tool that has as its primary use the management of texts and deltas of texts. You'd probably need to use a combination of tools which has its own set of frustrations.
Bitkeeper (the SCM that git was "inspired" from), claims good support for large binary assets
BAM solves this problem with a hybrid approach. BAM adds the concept of one or more BAM servers. Each BAM server contain all BAM data, but clones of the server (work spaces) contain only that data that the user requests. Typically this is the most recent version of each binary but in some cases it may even be less than that.
BAM has been successfully deployed in the game development space with great success. Game developers continue to enjoy the benefits of distributed work flow without the penalty of carrying all versions in every workspace.
I don't think you'll find a satisfactory tool that has as its primary use the management of texts and deltas of texts. You'd probably need to use a combination of tools which has its own set of frustrations.