Twitter extreme eg.--many of us non-users couldn't perceive benefits. Low barrier made it OK to say, "just try it..." Not true w/all things
This is a sometimes overlooked advantage of software as a service (SaaS) in its various forms. Even installing free or trial software can be challenging enough that all manner of virtual appliances and application virtualization have been suggested as possible solutions to this "pain point."
Of course, no barrier is truly zero height. Even signing up with a Web site, getting the hang of the basics, and (perhaps most of all) figuring out how or if it fits into the flow of your lifestyle and work don't just happen. This is especially true when the service in question is new and different. When it makes you approach an activity in a genuinely different way or otherwise shift an established mindset.
New is hard for developers and designers. It's also hard for users.
That said, the freedom to tell prospective users/customers to just press their browser at a URL and "play" is an incredibly powerful concept. Especially when the product in question lends itself better to experience than explication.
Kathy is right that Twitter is one such example. Before I gave it a serious run, I thought it sounded sort of silly. It was actually using it that convinced me otherwise.
Compare and contrast this to the case of TiVo and the digital video recorder (DVR).
TiVo changes how you watch television just as Twitter changes how some people communicate. Aside from some sports and news, I now rarely watch TV live. I almost never just watch "whatever's on." And I often don't even know which channel or night some program is on.
But TiVo the company has always had a great deal of difficulty explaining that transformation of TV watching. Especially early on, a lot of people viewed TiVo as essentially an enhanced VCR--when, in fact, the experience is qualitatively different. TiVo has been a tough sell to consumers because it required them to invest in a pricey piece of electronics for benefits that were hard to understand in the abstract.
DVRs in general only really started to go mainstream when they started to be bundled by the satellite and cable companies. In other words, when the acquisition barriers went down dramatically. And it's not even just about the cost, but about the mental energy and perceived risk associated with baking definitive choices.
Seeing sometimes is believing. But you have to make it easy to take a look.