A big barrier to public transit usage are the various things which add uncertainty and reduce convenience. Complexity in fare payment is a big issue, including uncertainty about how much to pay and precisely how to pay. My local transit authority is pretty awful in this area. It's ok for regular riders who buy monthly passes, though it's still frustrating that such passes are calendar month rather than 30-day passes, but pretty annoying for occasional riders for various reasons, including the shortage of token machines and complete lack of ticket machines for the regional rail system (they were scrapped because they couldn't read new bill designs).
So good news for Londoners that their Oyster card payment system will soon work throughout London - bus, tube, rail system, etc. From my brief experience, it's a model of how such card systems should work
Here's a map of the services it covers (.pdf). Works (soon) everywhere, can add money and buy passes online, and it caps daily cost at the cost of a daily pass automatically.
Though there's something oddly sad about the de-circling of the circle line...
