There were many tablets released before the iPad that did not sell that well.
Yes, Microsoft made them, they ran Windows, and since applications were not designed for touch they sucked compared to laptops.
What Apple did was not marketing, but make a tablet that was usage because everything from OS to software was made for a tablet, not a PC.
It also relied heavily on many IPhone developers being able to quickly write software for the tablet before it was even launched - we could only test apps on the simulator before they went into the iPad App Store on day one! Kind of insane if you think about it, but it generally worked because the devices were similar in OS. If there had not been a good base of software from day one, sales would probably not have been as good... oddly parallel to a console launch come to think of it.