There are a few potential disadvantages that I can see:
* It isn't necessarily a solution to the biggest issue for new/unknown authors: getting eyes on the page (or screen, in this case). Putting something on the internet alone isn't enough to get people to read, you still need to have produced something of quality (or something so bad it's funny!), have people find it, be willing to expend the effort to read, and then recommend/link to others. It is possible, but for any given book, exceedingly unlikely (what was the last novel you stumbled across online, read in entirety, enjoyed and recommended to all your friends?). You're competing with the rest of the internet for attention spans.
* Some people still consider something given away for free as being valueless (whereas of course we know that "price" and "value" are different, right?)
* You've exhausted your "first print" rights (electronic ones, anyway), something that publishers may still consider valuable. That is, to get published in the traditional manner after releasing free online, you'd need to develop an enormous following to make it worth a publisher's while.
* There is always the fear that "someone will steal my work". 396
I think the chance of this is happening in any meaningful way is vanishingly small, but it is a concern a lot of people have. People do copy and re-post things on the internet, so it definitely happens, but if your work was copied and then got popular from someone else, a) at least it's popular (which is really your goal?), and b) when the internet† sees something it considers "a wrong", it is quite good at investigating and outing the truth (a process which -- if it happened -- would likely generate an order of magnitude more publicity).
* Costs associated with production, hosting and maintenance (as <PERSON> mentioned above).
As a postscript, putting something online for free doesn't mean you can't make money. There are lots of business models you can look at: support via donations or "pay what you want"; popularity can open new opportunities for employment, expert comment, speaking, etc; selling physical goods to go along with the electronic (high quality print versions, for example). There are many others, I'm sure.
† Yes, I referred to the internet as if it's a conscious entity in it's own right. It is, isn't it? ;) 396