I've been a qualified ATCO for getting on for 7 years, working mainly in Class G airpspace so I've a little experience on this one:
PPL's and FI's - I lump these together as it amazes me at the difference in standards between one training provider and another, and the subsequent loading this has on my frequencies. Good training, especially R/T for the FRTOL is ESSENTIAL. Too many times have I had a student PPL or a low hours PPL say the wrong thing or do something silly because of indadequate or blatantly poor training. I do not wish to tar all schools/colleges with the same brush here, but I do wish standards as a whole would be raised.
I would counsel this - if they can't do the R/T to a good standard

- don't let them go on their qualifier (cross-country), PLEASE. I know practise makes perfect, but the place to practise R/T is not IN THE SKY but in the classroom.
As one colleague once said to me - Air Traffic Control must be one of the few jobs where you can lose your career because of someone enjoying their hobby....
As for Commercial Pilots - they can get annoying too (I know ATCO's can, before the comment gets turned around!) but especially when operating into a non-radar procedural unit in Class G airspace -
Guys do you not know why there is a difference between the callsigns APPROACH and RADAR? Do you not realise that (usually, Guernsey being a notable exception) an air traffic unit with the callsign APPROACH is working PROCEDURALLY - i.e. no radar? That means we cannot give you radar vectors to the ILS no matter how many times you

ask!!!
Also - we're not daft...although we might not be able to 'see' you all the time at a procedural unit, it doesn't mean we don't know what you are doing, especially when a procedure that should take 15 mins to fly takes 8...
Let's all be professional and give a professional service to each other, even if you are only a student/low hours PPL used to flying out of a strip with an air/ground radio service...it makes it a whole lot easier on everyone.