I suppose then that there are two sorts of departures, those who leave by their own choice and those who leave for other reasons (death, having your memory wiped by the Time Lords, or being trapped in a parallel universe). I tend to find the latter quite convincing: hence I’m okay with Rose’s leaving.
Leela was an interesting companion (apart from the obvious reasons) because whilst the Doctor obviously abhorred violence he sometimes seemed to recognise the need for it and turned a blind eye to Leela’s savagery. In some way they seemed to complement each other, so she didn’t (at least for some writers) just become the savage pupil/slave. For me, the culmination of this would have been Leela engaged in an act of violence to save the Doctor’s home world where he could not. Instead the Doctor’s actions seem quite out of character and Leela’s departure is a terrible let-down.
Anyway, I agree about some of the others: Susan’s departure fitted with her relationship with the Doctor which, while technically more parental (or at least grandparental) never felt like a permanent arrangement. Jo’s, as you say, was built up through the serial. Plus she always was a bit of an airhead and probably didn’t appreciate having the entire Universe to explore… Then there’s Teagan’s departure, which I found convincing (and irritating, like everything else she did). Otherwise, choosing to depart is always a difficult one (eg. Sarah Jane in both The Hand of Fear and School Reunion), and either RTD has to make it work or find a non-death way of removing the choice… I don’t have a problem with that if he can make it work, it just seems prophesising a single companion death when there wasn’t seemed ill advised.
And I’d forgotten about the guy from the library. That’s another loose end I was sure at the time would be followed up, and another missed opportunity…