Ask this question any time between 1967, when it was broadcast, and 1992, when it was released onto VHS, the answer from many would have been "Nothing".
Those who had seen that original broadcast informed those of us born just a little too late, that this was a perfect story in every way. This impression was reinforced by a very good Target novelisation of the story, written by its co-writer Gerry Davis.
For the benefit of those new to the classic era of the series, this story was one of the many lost when the BBC wiped the tapes, believing them to be of no further value. This junking of many B&W episodes wasn't widely known about, even by fans. Most believed the BBC had everything sitting in its archives. Come 1983, during the 20th Anniversary event at Longleat, a questionnaire was circulated asking attendees to nominate a 4 part story to be released on VHS. Tomb of the Cybermen came out top in this poll thanks to noone knowing it no longer existed. The BBC simply ignored this poll and released Revenge of the Cybermen instead - on the grounds that it was the only other 4 part Cyberman story they did retain, plus it was in colour and featured the most popular Doctor.
Tomb of the Cybermen was eventually found, in Hong Kong, in late 1991, and issued on home video the following year.
That is when fans got to discover that it wasn't as perfect a story as they had been led to believe...
The first thing fans picked up on were the very obvious visual problems, which you would never have known about from book or audio. One scene which people raved about was a Cyberman having its chest UNIT smashed, with horrible gunk oozing out. This scene had featured in a BBC Talkback programme about violence and horror on TV.
What we didn't know, was that immediately prior to this we get the Cyberman being held aloft over Toberman's head - with harness wires clearly visible. This had also been the case earlier, when Toberman himself had been held aloft. Towards the conclusion of the story we see Toberman lift the Cyber Controller - but this time it's clearly a substituted dummy, as we see the helmet almost fall off as he is thrown across the room.
We get a clear look at one of the Cyberman costumes coming apart under the armpit, at the foot of the ladder.
In a scene at the top of the ladder one of the Cybermen lets out an audible "Ooh!" as he is hit.
En masse, the Cybermen appear to quack like ducks when consternated.
Both the circular hatch and the door to the recharge unit, supposedly metal, are too obviously balsa wood.
Plotting issues include the good guys locking up the villains, unsupervised, in a room which just happens to have a powerful ray gun in it.
Ever seen an archaeological dig? Every inch of soil is sifted and every inch of surface meticulously recorded. Viner takes a couple of hand written notes and declares that he has every detail worth collecting from the recharging room. Not one of the party sports a camera.
Despite having met them on the Moon only recently, in this current design, the Doctor and Jamie are a bit slow to notice the big Cyberman images dotted all over the tomb.
The Cyberman plan is not as daft as some, but is still a little stupid. They've gone into hibernation due to running out of power and resources. Their tomb is designed to lure people who they can capture and convert. Making it really difficult to break into, and filling it with death traps, is not the best way to capture lots of new recruits to their ranks.
After getting up out of their tombs at the end of part two, the Cybermen just go back to sleep again in part four.
Why seek out only the most intelligent of interlopers, if you're only going to turn them into unemotional drones?
The Doctor seems to know about Cybermats, yet Jamie doesn't. The Doctor hinted that he might have met the Cybermen before his encounter at the South Pole, as he predicted their arrival after recognising their planet - yet he didn't know about their lack of emotions. He must have known about Mondas, but not about what happened to it's people, and his knowledge of Cybermats must come from an unseen story.
Not an issue at the time, but problematic now, is the representation of the human villains.
All three of the villains are of ethic minority origin, with one "blacked up" to play an Egyptian. Her partner is actually of Cypriot birth, but is playing another Egyptian - as he had done twice before in Hammer Mummy movies.
At least Toberman gets to redeem himself at the end and destroys the Controller (or so it seems) as the tombs are resealed.