I think people overlook how significant it is that The Doctor 'stole' the TARDIS.
TARDISes aren't like cars in Time Lord society. They're not accessible to all. Every Time Lord doesn't get the keys to a TARDIS from their parents when they turn 16.
TARDISes were used by the Time Lords to observe places in time and space in person, only when absolutely necessary.
Time Lord society, at the time The Doctor lived on Gallifrey, was isolationist. They were content to simply observe from The Citadel on Gallifrey. They did not interfere in events, much to the frustration of The Doctor.
The TARDISes were under the control of the Time Lord ruling body. They required several Time Lords to pilot it. Possibly due to the complexity of travelling through Time & Space. Possibly as a safety measure to prevent a renegade from absconding with a TARDIS and causing havoc with timelines preserved by the Time Lords.
The TARDISes were heavily gaurded. Stealing a TARDIS wouldn't at all be like stealing a car. Stealing a TARDIS, to put it in contemporary terms, would be closer to stealing a nuclear powered submarine.
Keeping the analogy, of an equivelant in our society to what The Doctor did, it would be as if a Senator plotted, and successfully executed a plan to steal a nuclear powered submarine on his own! Then used that nuclear powered submarine to travel the globe sightseeing, all the while avoiding persuit of his former government trying to reclaim their stolen property.
However, The Doctor didn't just steal the TARDIS, he also stole The Hand Of Omega ( as told in 'Rememberance Of The Daleks). The Hand Of Omega was a remote stellar manipulator, invented by Omega, which produced supernovas and black holes. So to continue my analogy, The Doctor didn't just steal a nuclear powered submarine, he stole one with a nuclear warhead!!
Side Note about the Hand Of Omega: Seeing as when The Doctor was finally caught during his second incarnation (and every subsequent trial) no mention was made of the theft of The Hand Of Omega, it is feasible to assume that The Doctor stole this from Gallifrey without any knowledge of The Time Lords. It has been demonstrated in several serials of Classic Who that, by The Doctor's time, Time Lords had lost track and meaning of some of the technology from the time of Rassilon and Omega. How The Doctor knew about the Hand Of Omega when others did not may be explained by the theory that The Doctor may be The Other ( a topic for another post).
Classic Who supplies hints that The Doctor was able to steal a TARDIS because the TARDIS he stole was a old model, close to being decommissioned and in need of repair. It stand to reason that perhaps the security around a TARDIS about to be decommissioned, a model that was already old when The Doctor was young, and one in need of repair - would have lessened security. Still, even decommissioned submarines that are only on display for tourists, have enough of a security detail that an average citizen on his own (or let say a man with his grandaughter) would have a lot of dificulty running of with one.
Niel Gaiman's 'The Doctor's Wife' also states that the TARDIS was looking for a Time Lord to steal herself. The TARDIS has always been described as 'sentient'. Perhaps the TARDIS did so out of a sense of self preservation. Knowing that it was likely to be decommissioned or stuck in a museum never travel in time and space again. Perhaps that is why The Doctor selected that particular TARDIS as the one to steal. He would require a 'cooperative' TARDIS.
Think of the sheer audacity and risk it would take for The Doctor to steal the contemporary equivalent of a nuclear powered submarine and a nuclear warhead!! This isn't the story of a silly man who ran off with a time machine...it's the story of a clever son of a bitch with balls the size of Pazithi Gallifreya (the copper moon of Gallifrey) who ran off with two of his governments most powerfull weapons.
For a Time Lord to do all that, well he'd have to be a 'Madman'.