I have been watching this thread with some interest.
From the training point of view (and, when all is said and done, line captains ought to be acting in a "coaching" role at least) you may have to decide whether to comment on one major aspect of a pilot's operation rather than drop on every bit of minutae. I think if you start badgering about every single item that is not quite "standard" this can become counter-productive and can lead to an unpleasant atmosphere on the flight deck. This does NOT mean that you should accept an unacceptable standard but this is all about HOW we encourage our fellow colleagues to be better at what they do.
Sometimes delaying making a point can be much more productive than dropping on it at the time. I recall flying with a good FO one day and we had to hold for a while prior to landing at Iraklion. The weather was perfect (day) and he let the automatics fly it round the hold at 250 kts. The max speed for holding was 210 kt but I decided to stay silent to see whether he might pick up his own error. He didn't but it was not until the next day over a cup of coffee prior to our next duty that I commented on it! He was most grateful and took the point on board. Remember that the foundation for the task is laid when you are NOT doing the task. Take a read of John Adair's book "The Skills of Leadership" where he talks about "action-centred" leadership. Generalising, in the "western" world we tend to be very "task" oriented whereas if we spend more time on the "group" and "individual" needs we are much more likely to get the task done efficiently.
Spend some time building a good working relationship with your fellow pilot and this will pay dividends. This does not mean we have to go all "touchy/feely" just use common sense and establish some rapport before addressing the task issues! This takes extra "work" but in the end is much more satisfying and productive.