In this question all humans have had this ability for as long as there have been humans. I've also posted a separate, different, question where the power is exactly the same, but all of humanity suddenly gets the power one day. See meta regarding the split.
The actual question is very concrete and comes at the end, but first, I must explain the premise in detail.
This world is exactly like our own, except for one (massive) difference. Everyone can know what everyone else is thinking. In this world, anyone can access other people's memories, thoughts, plans, ideas, beliefs, etc. This is not a technology, it's more like a telepathic power that everyone has and cannot lose. The ability is at least as natural to everyone as using the five senses is in our world. Anyone can do it.
You don't need to know much about anyone to hook into their mind. For example, you can sit in a hotel room, and think "Who painted this wall?" and immediately access that person's mind in an unlimited fashion. Or you can think "Who is thinking about me right now?" or "Who is climbing Mount Everest right now?", and get access to their minds, too. It's almost like a perfect, intelligent search engine. There are almost no limits to this.
There are, however, some limitations:
In the case of the painter, if the painter is dead or doesn't remember painting that room, you'll be unable to find an answer to your question. Unless someone else currently alive knows who painted the room (in which case you can still not access the memories of the dead painter himself). So, you can only get information that currently living people still remember.
The main limitation is your own mind. There is way too much data to process, so you have to select what you focus on. It's a little bit like the public parts of the Internet. You have access to everything, but a single person couldn't possibly process all of it. They have to choose what they prefer to focus on.
Even though you can sense the thoughts, etc, you don't feel that they are your own. You observe them with a distinct impression of them coming from outside of yourself. People with more empathy may feel more compassion or connection with what they sense, but in a similar sense that they would feel empathy if they saw a person suffering in the real world, through the senses. The degree of empathy depends on the individual, but it may be enhanced by the greatly improved ability to understand others.
Except for the (massive) implications of this ability, it's the same earth, and the same humans.