Which shows are you thinking of?
I've already named a few earlier in the thread - The Prisoner, The Twilight Zone, and Star Trek. Let's leave personal preference out of this if we can, and try to stick to facts as much as possible (I don't want to derail this thread with a Doctor Who vs. Star Trek argument; I actually prefer watching classic Doctor Who to original Star Trek.)
First, the pace of Star Trek is far more accomplished than Doctor Who, objectively. Much more happens in 45 minutes of Star Trek than often happens in 2 hours in Doctor Who. Even a mediocre or average Star Trek episode is rarely boring, and the scriptwriters have a good handle on the 5-act structure of a television show. They know how to increase tension, decrease it, and then increase it in time for a commercial break, and those commercial breaks are far more dramatic and varied and creative than your average Doctor Who cliffhanger (imagine if, in Star Trek, at every commercial break, someone were about to kill Kirk or Spock, or execute them, or throw them off a cliff, or crush them in a complex machine, or shoot them - the 60's Batman and Robin series understood that this would be comical and silly more than the writers of Doctor Who ever did.)
Second, the main characters on Star Trek show a lot more personality, levels, subtlety, and depth than the main characters on Doctor Who. Kirk and Spock are far more complex than the Doctor, far more like real people, and McCoy shows far more personality than even the best Doctor Who companions (Romana, Sarah Jane, Ace, etc), with the exception of Ian and Barbara, who are fantastic, and just as realistic and complex as any Star Trek character.
The conflicts on Star Trek are often internal, not just against an outside threat. Kirk often has to debate within himself what he ought to do - should he break the prime directive? Should he let Edith Keeler die? Should he trust Khan? Should he allow Spock to go back to his planet to mate? Should he ignore the commands of a superior officer? Even in the mediocre episodes, there is almost always an internal conflict with one or more of the characters. In Doctor Who, that is very rarely the case. The only major internal conflict I can think of is the Doctor wondering if he has the right to destroy the Daleks before they have been created, but that conflict lasts about 15 seconds, and should have been at the heart of the entire episode.
I could make the same comparisons with The Prisoner, another early sf show that has an excellent understanding of character, conflict, and pace.
Anyway, my point, believe it or not, is not that Doctor Who sucks. As I said, I actually would rather watch an old episode of Doctor Who than an old episode of Star Trek. I guess I just don't understand why Doctor Who, despite the fact that the writing is more often than not pretty sloppy and amateur, that it entertains and entrances nonetheless.