It will be interesting to see how quickly the Y14.5-2009 will be adopted.
If you have a Y14.5M-1994 drawing, its interpretation would be almost no different though the new standard has clarified much.
The 2009 standard has some extra "goodies" such as movable datums, and some new symbology tools (e.g. continuous feature & spotface symbol).
The big advantage of the 2009 standard is that the spec is more clearly written, the nature of datums and their orientation has been clarified (related and unrelated actual mating envelopes), and composite tolerancing has been expanded.
If you read them both cover to cover (I have), the 2009 spec is a much clearer read, and what was fuzzy before (e.g. orientation of mating envelopes), is now often clearer.