I think the worst one on the list is actually positive thinking. I've had many discussions and arguments with people who believe that one just can think themselves out of a serious life threatening mental illness for which science up until now has no cure. Often these are people who believe in the law of attraction or who just heard of the word neurogenesis. And of course there are the people who had some minor depression at one point in their lives (something like heartbreak for example), came out of it through whatever means and then believe that they discovered the cure for all mental illness. It's just not possible for them to understand the difference between a light depression and a chronic mental disorder. There would be no need for shrinks or drugs etc. and many suicides could be prevented if we could just think happy thoughts and be done with it. Another argument along those lines which I absolutely hate is that depressed people just want to be depressed because they take delight in the negativity etc. One would almost wish depression on the people making these claims, just to see how well they would cope suffering from extreme depression.

Vitamin supplements (in case someone doesn't have access to healthy foods and/or sunlight) as well as exercise and meditation are actually good advice in my honest opinion. If someone believes that these can cure a serious mental disorder though, they might be far off - although I'm not sure about meditation, since I meditated about 8 hours a day for days on end and felt absolutely marvelous. The problem with that is, it's not only very time consuming but these higher states of mind (exacerbated by the chemicals released in your brain during meditation plus the rapid changes in brain-structure) make it very hard to function in day to day life. So unless someone wants to live the rest of their lives like a Buddhist monk this isn't exactly a solution.

And like you say in cases of deep depression where it's impossible for people to get themselves out of bed, this advice, including exercise, is absolutely worthless.

Small steps are the key I think, not just if it comes to exercise and meditation, but pretty much with everything when living life with a mental illness.