This seems to have been largely ignored by the media (and was even quite hard to find on the BBC London website), but Boris has bought the last remaining tube maintenance contract in house.
One of the major problems with the previous arrangements is that they allowed the companies to force TFL to close tube lines when they wanted, which has caused a *lot* of disruption to London, and potentially a lot of damage to London's image as a modern, efficient city. Damage that could potentially cause a lot of jobs to be lost in London.
Take, for instance, the Jubilee line upgrades. These have meant that the line has been (at least partially) closed nearly every weekend for nearly two years. According to a programme shown on the BBC, similar upgrades on one of the Paris Metro lines (a line that is a similar length to the Jubilee) took 1 year, and involved no weekend or bank holiday closures. The difference being that the Paris Metro work took place overnight. Something which TFL apparently claim is too expensive.
http://news.bbc.co.uk/1/hi/england/london/8672312.stm
The major thing Boris has promised is fewer closures.