Once, I played a virtual reality game where robots attack you and you shoot them. This was a very fun and compelling experience despite being rather basic. In fact, it was too compelling -- the experience was so intense that I had to stop playing, as I thought that if things got significantly more intense I might have a heart attack and die!
This experience got me thinking something along the lines of: "Huh, virtual reality technology is rather primitive, but already an experience can be that intense? What will it look like once virtual reality environments have much more time, thought, and technical advancement put into them?" The prospect seemed pretty worrisome to me for the future of the technology and how it might impact society and civilization. (Of course, fears of negative ramifications of virtual reality are by no means new, but having a direct experience of it made me put more thought into the matter.)
However, I brought this up in an online conversation recently, and another user, ohAitch, made a very interesting point in reply -- many of our friends spend huge amounts of time on textual fiction, such that virtual reality might well not be a big important step!
This made me realize something -- for many people, these dangerous virtual worlds are already here. For some people it is the virtual socialization of Facebook or Twitter. For others, it might be non-VR games like World of Warcraft or Counter-Strike -- there have been multiple cases of people dying at their seats after playing games for hours on end in Internet cafés. For others, it can indeed be textual fiction as ohAitch said -- webfiction, online fandoms, and the like can be very consuming.
Yes, virtual reality could certainly be immersive and dangerous in a way that some of the earlier things haven't been, could make it easier to fall down a slippery slope, and so on. I think there's a real threat there -- but there are also some threats that are already here and which it might be prudent to pay attention to. The risks of dangerous lotus-eating are real and present today, and if we only worry about what future things might bring we may miss the dangers that are already around us.
 I personally know people whose lives have in my view been substantially worsened by Twitter politics in particular.