Quattro,
On the barriers to entry and skill things leading to higher pay...
You're a solicitor I think (not stalking, I think I just read it lower down in the thread...?); I'm an academic; doubtless we both have friends who are doctors.
All three jobs have high barriers to entry, are costly in cash terms to the trainee for considerably longer than the average degree, require enormous (state) investment in providing high level skills and specialisms. You said yourself, your own DH working in the public sector has a similar profile and training to you but earns less (I think).
The variability of pay across these three types of job, and within them (since there is considerable variability in salary dependent on types of role for two out of three of these posts) cannot be explained by barriers to entry, training and skill.
So to use this as a counterpoint to say nursing or caring jobs don't get paid as much because of the training and entry process doesn't really work for me. The key difference between the nurse and the doctor is probably professional responsibility and liability (notwithstanding the fact that in practice, nurses are increasingly being made responsible for clinical practice and decisions); the key difference between the legal secretary or clerk and the solicitor is what - I dunno enough about the job - but something similar along lines of level/remit of work but also legal responsibility for outcomes and perhaps financial commitment if partners etc? In my field the key defining role is probably based in degree awarding powers (and believe it or not, this is a pretty big and increasingly litigious responsibility) and some notion of academic freedom (in comparison to more independent knowledge creation).
So I think there are differences between professional jobs and other jobs, but I don't think any of skill, training, hard work, commitment, financial sacrifice are in themselves necessarily a 'cause' of higher pay.