Required:
- open source license
- easy to install on all major distros
- low overhead for large number of repositories (one per document)
- fast for simple operations (create repo, checkout any version, commit to any branch, create branch)

Useful:
- good documentation
- easy to set up / use
- good delta compression
- low-level bindings (Python or at least C)
- atomicity / "global" (per repo) version ids (so we can store entire directories)

Don't care:
- rename support (filename is artificial)
- copy support
- repository mirror support (internal usage only)
- change propagation to parent repository
- permission support (handled by higher level)
- changesets/transactions (atomic groups of modifications)
- blame (not useful for most of the files anyway)
- partial checkouts
- per-file commit message
- networking support
- portability (to non-POSIX)
- web interface
- GUI


VCS				OSS[1]	install		overhead
----------------------------------------------------------------
RCS				GPL
CVS				GPL	+
AccuRev				-
Aegis				GPL     -f
GNU Arch			GPL	DFU
Bazaar				GPL	DFU
BitKeeper			-
ClearCase			-
CM+				-
CMSynergy			-
Co-Op				-
Darcs				GPL	DFU
Fortress			-
Git				GPL	+
LibreSource Synchronizer	Qt	-d
Mercurial			GPL	DFU
Monotone			GPL	DFU
OpenCM				GPL	-d
Perforce			-
PureCM				-
SourceAnywhere			-
Subversion			BSD	+
Superversion			GPL	-d
Surround SCM			-
Team Foundation Server		-
Vault				-
Vesta				LGPL	-d
Visual SourceSafe		-


[1] http://better-scm.berlios.de/comparison/comparison.html