Milt makes some valid points but I think there is already some standard phraseology that does the job. In the UK it's 'Go around, I say again, go around, acknowledge' - I'm not sure if that's straight out of ICAO but if not I suspect it's pretty close.
The problem arises when there is no need for the pilot to hit the TOGA buttons or whatever because, say, the aircraft is still at a safe level and simply needs to be repositioned onto the approach for some reason. To instruct a go-around in this case seems unnecessary and is potentially confusing to the pilot.
Hopefully most controllers will recognise that the crew may have their hands full if the go-around is initiated by the pilot, but even so, in some of our high traffic density environments it may be essential to pass additional instructions to the crew at a busy time. In this case I would limit the instructions to a heading and level - which in most cases will suffice. These considerations still come into play for an ATC initiated go-around but, again hopefully, the crew will only be having to contend with a 'normal' go-around.
The issue of pilots manual handling skills is, I suspect, starting to show itself by the reluctance of some crews to accept clearances that require manual handling, or by SOPs that precluded them from doing so, which then further degrades their skills - visual approaches immediately come to mind in this context. Having said that, from what I have seen during liasion visits etc. those operators/crews who 'rely' on automatics for normal operations also rely on them to a large extent during a go-around or other non-normal event - I'm not suggesting that this is a bad idea but it further exacerbates the issue that Milt has touched on.