MY OPINION on the differences between a professional and a really good amateur from a musical standpoint.
A professional:
- Is on time
- Is prepared
- Has all the gear necessary for their performance
- Knows how to set up and use such gear
- Might require some rehearsals, but they are productive and have a clear goal. Often, rehearsals are for large performances; smaller gigs played by professionals do not require rehearsals.
- Is concise when talking to bandmates; doesn't send a million emails or texts
- Knows the ins and outs of the music industry
- Presents themself in a polite manner
A really good amateur (usually):
- Is on time
- Is prepared (or mostly prepared)
- Has most (or all, or none) of the necessary equipment
- Doesn't really know how to use their gear
- Requires several rehearsals before the performance. Rehearsals are not as considerate of bandmates, and are often unproductive.
- Demands things that a professional wouldn't
- Sends out far too many emails and texts, often convoluted and haphazard
- Knows the music industry somewhat, or enough to make good judgements
- Presents themself in a polite manner
Obviously experience is one glaring thing missing in this list, but I don't think it's all about experience. I think the above factors are equally, if not more, important.
I think that you should add "experience" to your list, and recognise that (neither) list is complete, as there are other variables that could be added to both lists.
In any case, experience tends to give both confidence - and competence in such a context, that is, competence, the sort of competence that comes from frequent playing under the pressure of performing in public, and also, above all, in timing, (timing the pieces, and playing the pieces) - for playing in public is quite different to playing in private, from a psychological perspective.
And experience tends to give you better judgment (re yourself, your instruments, your colleagues, above all, if you have played with them before).
Playing in public (whether as an amateur, semi-professional, and/or full professional), also gives you a markedly different relationship to time, and not just in rehearsals (which is where you learn how you work with other musicians, and their quirks and foibles, as musicians and people, it is always easier to work with those whom you know - and whose playing one knows, than it is to work alongside someone you don't know, and whose style one has yet to become familiar, or, at ease with), but also to a deadline, for, professional performances are invariably performed to the clock (a set time on stage - and, one that must be adhered to, to the minute, if the perfomance is being broadcast, especially if it is being broadcast live on radio, or TV).