As EnglishMan has said, watch the two recent seasons with Christopher Eccleston and David Tennant as the Doctor; they are shot on high resolution digital and look amazing, with a "cinematic" feel to each episode and superb fx... indeed very distinct from the 'classic' series...BigJohn9x8 said:3) It was shot on videotape and therefore looks CHEAP!
I think it's #3, the cheap videotape look to the series.
However, also check out Spearhead from Space with Jon Pertwee from 1970... which was the very first Doctor Who shot in color [http://www.bbc.co.uk/doctorwho/classic/episodeguide/spearheadfromspace/]. Due to industrial action by BBC technicians, the whole thing was shot on 16mm film with considerable location filming ... and looks frankly incredible.. almost like a separate movie spin-off to the series, and as such, entirely in a different league to the other shot-on-video Pertwee adventures. It also has a considerably faster editing rate, meaning that the longeurs of the many talkathon scenes in the shot-on-video episodes are absent, with the action really moving apace. It may well not be coincidental that the first episode of the revived series starring Christopher Eccleston was to some extent a remake of the same story.