The Time Meddler marks a few occasions. The first is the departure of the last of the original companions. As mentioned, Ian and Barbara took their leave at the close of the previous serial, The Chase. There is a new set of companions now, neither of which are using the then-modern day as a reference. Vicki, Susan's replacement after The Dalek Invasion of Earth, was from the 25th century and Steven is from approximately the 23rd. That opens up some theoretical possibilities. Finally there is the titular occasion of our first meeting with another member of The Doctor's race besides his family, and a whole new type of Doctor Who story.
The Doctor, Vicki and Steven arrive in England in 1066 and it takes a great deal of convincing Steven that they have indeed travelled through time. Not helping the case are anachronous artifacts, like pocket watches and gramophones. It seems that another time traveller is playing a wicked game. Queue The Meddling Monk. Though the name of their species is never stated (and would not be until the end of the Second Doctor's reign), he is another Time Lord with another TARDIS. He also has a plan involving the Viking invasion of England.
These elements mark The Time Meddler as the first pseudo-historical of the series. To this point, the strong Science Fiction content was limited to stories set in the future or on alien worlds. Of the twin educational goals of this children's show, science was for the future and history was for the past. There was no time to involve The Doctor and his companions in Sci-Fi antics in antiquity because there was so much to teach.
At the close of the second season, with the original cast gone save for William Hartnell himself, producers were ready to try new things. It did take some convincing however. The public could not, at first, make heads nor tails of the pseudo-historical. Toasters and gramophones amongst the early Britons indeed! Yet The Time Meddler would become the first of many. The remainder of the First Doctor's run would have another four straight historicals (or, more accurately, three straight historicals and one musical Western), but afterwards, pseudo-historicals like The Time Meddler would take over for any adventure set in the past. Nowadays, Cyberkings in Victorian England and Daleks in the London Blitz are the norm and historicals are unheard of.