I'm watching 'The Time Monster' tonight. It's the first time I've ever actually seen the story, which has been described as "like watching paint dry while being flogged with barbed wire--immensely dull and painful at the same time" by the Discontinuity Guide. About Time wasn't particularly charitable to it either, nor was Who's Next, nor were any of the other episode guides I've read. The fan consensus is that this is an embarrassment at best and a travesty at worst, the sort of thing that must be hidden from the Not-We lest they forever flee Doctor Who.
In actuality, it's not bad at all. It's a typical Pertwee story, overlong and relying rather heavily on the personalities of its leads (particularly Delgado, who can carry just about any load of old tosh simply by being himself, and who is allowed to have that rarest of all commodities in Doctor Who villains, a sense of humor) and with special effects that can't carry the story one whit. But in practical terms, it's no worse than 'The Silurians', and it's at least shorter.
Likewise, last week I watched 'Timelash', another contender for "Worst Story Ever", and was surprised at how entertaining it actually is. It's insanely overambitious for its production budget--the Timelash itself, particularly its interior, probably couldn't be realized on today's Who budget, let alone the cash-strapped mid-80s series--but the script more or less holds together, Herbert and the Doctor have magnificent chemistry together ("I hadn't realized that all this dying heroically would be so hard on the nerves") and for all the complaints about being OTT, Paul Darrow plays his part exactly as he needs to. Playing Tekker as "subtle malice" would have resulted in him being blown off the screen by Colin Baker's coat, let alone the man himself; this was a character that needed to revel in his own malice, glorying in his triumphs and the reflected glory of being the Borad's right-hand man. Tekker is a man on top of the world and loving it. That works for this story.
But the point of all this is, if you'd asked me what I thought of these stories beforehand, I'd probably have told you they were both awful. I'd have been honest, and clarified that I was repeating received wisdom in the case of the former, but the brutal fact of the matter is that it is human nature to find our memories and opinions gradually shifting and adjusting themselves to fit with consensus. Psychological experiments have proven it; when we are wildly at odds with the group opinion, it's hard to retain your own feelings. You convince yourself that maybe you just didn't notice the things everyone else did, that you were being uncritical (or overly critical) about something new. Fan consensus ossifies and hardens, becoming a permanent part of the lore (like "Empire is the best Star Wars movie", or "The even-numbered Trek films are great and the odd-numbered Trek films suck".)
And that's why it's worth watching these old episodes again, even though we've seen them before (multiple times, in some cases.) Because the text stands alone, apart from fan consensus and received wisdom and all that other stuff that we bring to it. When you watch 'Timelash', everyone's opinions of it suddenly become magnificently irrelevant, because you're seeing the actual story. Maybe in a few months' time, I'll lose that revelation, because it'll be hard to withstand the repeated slings and arrows of people who were expecting 'Star Wars' on a 'Doctor Who' budget, but that's what the story is there for. To break up that bony shell of "what we know" with "what we see", and rediscover the stories all over again fresh.