I submitted an enhancement request a while ago when it became very obvious that troubleshooting SAN path problems with the native DM stuff was pretty lax. Coming from a background where using VxDMP or PowerPath utilities to figure out whether paths are getting identical traffic, have queued IOs, etc. is way, way easier in those environments than in RHEL, at least as of 5.x. Maybe 6.x has solved them, but as far as I know the request is still "pending". I say it's high time to make DM more robust and give the SAN guys the real tools to figure things out without some custom one off script for every system out there which parses iostat, SAR, or whatever to pull all the pieces together.
The other thing is the somewhat mediocre integration with the SAN arrays, namely EMC. I'd think the interrogation or interaction with the array for either snapshots or NDU type activities would be well worth the time.