Yes the timeframe of needing the money makes a big difference to invest in. I am hoping my investment in stocks is for the short term. I prefer to invest in other assets.
I own part of a teak tree company which has a very long term outlook. No one take this as advice to do the same as there are some very bad investments in this arena. I am interested in rental properties and raw land but it takes a long time to get the capital together. That is where stocks come in. Very easy to get in and out.
Stocks are easy but I do think people would be better off investing in other assets. Corporations have a lot of overhead and insider trading. But the mindset is stocks and bonds as a lot of people are supported by it.
My next house will have solar.
Diversity is good, along with the general caution of invest only in what you understand, beware speculative businesses (not necessarily Avoid, just perform your due diligence). Indeed, stocks are very easy to buy without heavy minimums nor large broker fees, not too difficult to buy bonds but bond funds very easy to buy. For treasuries, treasurydirect.gov lets you cut out middleman.
I personally want no part of real estate beyond my home (I'm not suited to it), so I buy REITs and let someone else be front lines. I only have them in retirement accounts, bringing up another point of hold your assets in the correct type of account. For example, tax exempt munis don't belong in your tax shelters, but TIPS do. Stocks can go anywhere, just understand taxation of divs and gains/losses, a gentle reminder for anything you own!
One REIT I like, Realty Income Corp (ticker O) is "the monthly dividend company" and I happen to be partial to getting paid as frequently as possible.
As a previous poster said, I do not want to discourage anyone from their well-thought-out-plan. Mine is to stay the course with quality div paying stocks until the end of life and let my heirs continue to benefit from quarterly payments. I'll chuck the dayjob when monthly divs are higher than paycheck (possibly 7 years, more like 10 based on conservative projections).
I expect that sounds way too risky to most folks but it is entirely suitable to me and keeps market watching, agonizing decisions and fees to a minimum. I own over 70 companies at this point, some held for over 20 years. Probably they won't all last my lifetime but I expect that and would sell a company whose earnings fail to continue increasing. It is less work to own an ETF but less profitable and less control. Wanting control, I'll put the time and effort in to be able to project my quarterly income per company and own only the companies I want to own. I'm buy and hold (so far) but that does include monitoring.
I am not one that will depart stocks before/at/in retirement although I will hold more cash starting one year before retirement.
I would recommend reading Lowell Miller's The Single Best Investment for anyone interested in owning stocks. It is an easy read with great concepts and I believe first appeared in the late 90s. free pdf:
http://www.mhinvest.com/files/pdf/SBI_Single_Best_Investment_Miller.pdf
