As someone who gives this "advice" all the time, I know it isn't "advice". My issue is that people seem to have the assumption that becoming great at something is about following some set of steps that can be explained: read this book, take this class, or drink this magic potion.
Instead, the reason people I know are great at the things they are great at is because that's all they do: when they wake up in the morning the first thing that pops into their mind is to grab their computer or violin, head to the library or laboratory, and be "doing something".
These people don't often have much else in common: their methods may even look like madness, with atypical technique permeating their self-taught landscape of awesome. There was not this "moment of hesitation" before they jumped in to do something.
And to me, nothing demonstrates this "moment of hesitation" better than someone who isn't doing something because they feel they aren't ready yet. I've seen people sit around for years, somehow operating under the principal that eventually the magic potion will appear, they'll drink it, and be great.
My contention is that these people will not become great. Now, I /don't/ believe this is incurable, but I believe that it requires someone really passionate about what they do to try to explore that passion with them: to help break down those walls that are keeping someone from "already" being great.
However, even more often, I think these people are in the wrong field, and here is where I believe "be passionate" goes from platitude to "advice": it is the advise to do something else that you are /actually/ passionate about instead of the thing you are currently sitting around talking about.
So, if you are currently languishing trying to build a company: you don't see how other people can spend the time they are doing, you can't bring yourself to occasionally make a fool out of yourself in a negotiation... you simply consider the thing "work" as opposed to "the most important thing to be done when you wake up in the morning", you are starting the wrong company.
This might involve choosing a different area, one you really believe in, but it may also just involve removing road blocks, whether they be external or internal, from the problem so you can relax: maybe you need an assistant, someone who has a passion for organization, to help you out with things you find boring.
Here is where you may then say: "so the real advise is get an assistant; saying be passionate is just some weird cop out". No: being passionate was the only concrete advise; it is the core belief you can use to rapidly take your issue and convert it into an action item. You start with "be passionate", identify why you aren't, and then solve it.
Now, the stupidest part of this is that I actually "agree" with this article, but I still think the author totally missed the point: I'd say this is the checklist used when you /are/ passionate about something to determine "is this a business".
Just because you are passionate about something does not mean you will (or even can) make money doing it. As wccrawford says, if you aren't putting your all into it, you are all the more likely to fail, and the thing that is going to allow you to put your all into it, day after day after painful horrible day, is "passion".