Okay, speaking as someone who started with the old serieses (?!), I think it's fair to say that the old series were cheaply and shoddily made - BBC special effects = cardboard & flash powder, mainly, with a wide range of actors some of whom were actually good actors, many of which were from the pool of unemployed British actors.
The elements which lifted it were, IMHO, the imaginations of the writers, the opportunity to re-use existing BBC historical sets & costumes, the recurring story elements (Daleks, Cybermen, the Master in later old series) - remember, "light entertainment" hadn't discovered the story arc at that point. And above all, the strong (overacted?) characterisations provided by the key actors playing the Doctor. A lack of serious competition - Star Trek came and went, and only lived on in syndication.
The latter series - McCoy and later - suffered, again IMHO, from weaker actors, greater competition, and (towards the end) weaker writing. YMMV, of course. Post Star Wars there were other TV sci-fi stories being shown, and DW suffered by comparison. Consider Blake's 7, from a similar point in time - same BBC cardboard, a better set of spaceship effects, and strong ensemble acting by Blake, Avon and Servalan, amongst others.
