SERVER-5290, mongod correctly returns an error when trying to index a key over the maximum allowed length. There are plenty of workarounds for new applications (hash the value, truncate the value), but for existing applications with millions of documents, this causes major issues.
Allowing users to relax this restriction will permit them to gradually deal with these bad index entries. They can use the upgradeCheck tool (
SERVER-8391) provided in 2.6 to get an idea if this is something they need or not.
To enable this flag starting in 2.6.0, start mongod with --setParameter failIndexKeyTooLong=0. It defaults to true.