Some specific issues:

1) on the command line one types --dbpath. in the config file, it is dbPath:. lowercase isn't supported. that may be wise, however i suggest the error message if you typed the tolower of a valid parm name should be " ... , did you mean dbPath:?"

also if i have a tab, intentionally to test the error message, i get this:

Error parsing YAML config file: yaml-cpp: error at line 2, column 8: illegal map value

2) does illegal map value actually mean something to the user? (i don't know yaml well.) would "unexpected character" make more sense? the position is given which is nice.

3) can we just scan the whole file for a tab up front and say "tabs are not legal in yaml" as that is going to happen A LOT (if we don't want to just accept them as equivalent to a space, even though YAML doesn't...)

4) If I have an unknown option in a sub-section, the error message returned to the user says that the top level section is unknown. e.g.,

dan:(git)mongo[master]/$ ./mongod -f f.cfg
Unrecognized option: replication
try './mongod --help' for more information
dan:(git)mongo[master]/$ cat f.cfg
replication:
    master:true
dan:(git)mongo[master]/$ ./mongod -f f.cfg
Unrecognized option: replication
try './mongod --help' for more information