After years as a punchline, virtual reality is enjoying a renaissance. Venture capitalists plowed $1.2 billion into virtual-reality and augmented-reality startups in the first quarter of 2016, according to a report by Digi-Capital—more than ever before. The headliner was a massive $800 million round for a secretive “mixed reality” startup called Magic Leap.

You might assume that the early VR pioneers and advocates who suffered through the mockery and the doldrums would be rejoicing. And they are glad for the attention, not to mention the money. But they’re also worried. They’re worried that the companies and investors driving the trend are missing the point—and that, without a course correction, all the hype around virtual reality could lead to disappointment once again.

One of these skeptical advocates is Jeremy Bailenson, a professor of communication at Stanford University and the founder and director of its Virtual Human Interaction Lab. It was his lab that Mark Zuckerberg visited in 2014 to explore VR’s potential firsthand, two weeks before he announced that Facebook would buy Oculus VR for $2 billion. Bailenson’s research on virtual reality’s social and psychological effects has helped to shape the technology’s use by journalistic organizations such as the New York Times and the Guardian. Yet, for a man so instrumental in VR’s second coming, Bailenson is not particularly optimistic about its chances to revolutionize consumer technology, entertainment, or the media, at least in the short term.

The key to understanding virtual reality, Bailenson said, is that it’s both potent and taxing. It’s potent because it’s so visceral and immediate that it can trick your brain into mistaking it for reality. In doing so, his research shows, VR experiences can change how people think about themselves and others, perhaps in places where other media might fail. In a just-published study, Bailenson and colleagues found that taking the perspective of a cow or even a coral reef in a VR simulation produced more empathy for those organisms’ plights than watching a video about them. In a 2013 experiment, people who experienced colorblindness through a VR simulation went on to spend more time helping people with colorblindness than those who merely imagined having the condition.

Yet that immersion comes with costs—physical, mental, and financial. Though the technology today is far better than the laggy, monochromatic Nintendo Virtual Boy games that gave people headaches in the 1990s, virtual reality headsets can still fatigue your visual system, he says. VR content can also take a psychic toll. Shooting up bad guys on a video-game console is one thing. But “when you’re using your hands for murder, and you’re feeling haptic feedback as blood spatters on you, it’s just a different category.” Similarly, “if you saw Jaws in virtual reality, you might never go in the ocean again.” It’s not just that users risk being desensitized to violence, although that is one possible effect. It’s also that violence in virtual reality can simply be exhausting and unpleasant.

And there’s a third, more practical tax: the tax on your attention. Playing games on your phone or TV can be a time-suck, but you’re still at least marginally aware of the world around you. Strap on VR goggles, and you’re effectively blinded from reality, to the point that you don’t know who’s in the room with you or whether a car is about to hit you.

It’s also wildly expensive to create. “Movies only have to account for the small window of space where you point the camera,” Bailenson notes. “Video games are not infinite. But VR has to work from any distance, any angle, all the time, so the cost of making content is astronomical.”

What, then, is VR good for? Bailenson is glad we asked.

“Given that immersion is not free, I believe we should reserve it for special experiences,” he says. His own lab has an informal “20-minute rule” for VR content. Any experience that requires the user to go longer than that without a break is frowned upon.

In particular, he has come to believe that experiences worth creating (or recreating) in VR are those that could be described with one or more of the following four adjectives:

Expensive: If it would cost a lot to do something in real life, like visit the statue of David in Florence, Italy, it might make sense to do it virtually.

Dangerous: Climbing Mount Kilimanjaro might be the adventure of a lifetime, but it could also be your last. Several people die on its slopes every year.

Impossible: You can’t travel back in time, grow a third arm, or experience life as a person of a different race or gender. But VR can give you a surprisingly visceral taste of what it would be like if you could.

Rare: You could go whale-watching a dozen times without seeing a humpback breach right next to your boat. Or you could do it once in VR.

Expensive, dangerous, impossible, rare. As a mantra, it’s reminiscent of the “three D’s” of applied robotics, which suggest that robots are best suited to tasks that are dirty, dangerous, or dull. These types of constraints serve to rein in short-term expectations for a trendy technology that’s easily overhyped, while guiding it toward some promising purposes in the meantime. The robotics industry today is less about building a real-life Rosie Jetson and more about automating unsavory tasks. Likewise, Bailenson believes virtual reality will prove more valuable in the workplace than in the living room.

While Bailenson has been propounding this rather unsexy vision of virtual reality for years, it’s worth noting that he now has a vested interest in it as well. In January 2015, he cofounded a VR startup called STRIVR with a pair of former Stanford football players. The company produces VR training software for athletes, and in its first two years it has signed up dozens of professional and college sports teams as clients. Bailenson told me STRIVR recently landed a deal with a major retailer to expand into employee performance training.

Bailenson’s criteria don’t explicitly rule out movies or games as a vehicle for VR experiences. But they imply that even the most basic movie and gaming conventions—world-building, character-drawing, shot-framing, plot exposition—are inessential to this new form. In fact, VR as a medium is a singularly poor fit for linear narrative: The viewers’ ability to look in any direction makes it difficult to focus their attention on any given plot point. “You see these 20-minute VR film trailers emerging for things like Star Wars or The Martian, or stories being told in VR journalism,” Bailenson says. “The market is trying to use the template they have for their old mediums and bring it to VR.”

Jaron Lanier, the VR pioneer who coined the term virtual reality, agrees. “People who are on the inside of VR mostly feel the same way [as Bailenson] about movies and gaming,” Lanier says. “And yet because there’s so much enthusiasm from the movie and the gaming world, and also so much money, it’s a little awkward to say that. But it’s true. They both just seem to really miss the point for people who are experienced in VR.”

Lanier says he’s particularly concerned about the misogyny and “general weenie-ness” of gaming culture carrying over to virtual reality. He’d prefer to see it evolve into a platform for creativity and social interaction. But, he adds, “this is probably just the normal course of things.” He notes that a lot of early movies were essentially just recorded stage shows—it took time for cinema to find its niche. Meanwhile, Lanier says he’s encouraged by the success of Pokémon Go, an augmented-reality game that compels people to leave their basements and engage with the neighborhood around them.

Like Bailenson, Lanier has his own stake in the technology’s development. Among other endeavors, including a post on an EU data ethics advisory group, he’s an interdisciplinary scientist with his own lab at Microsoft Research, whose parent company is developing an AR device called HoloLens.

Lanier may be right that it’s typical for a new medium to struggle with the ill-fitting conventions of its predecessors. But that doesn’t help all the investors and companies that have placed big bets on virtual reality games and moves. “For tech firms to make money, you have to have eyeballs on this stuff more than a few minutes a day,” Bailenson says. “The problem in my opinion is that this technology is not an 8-hour-a-day thing. It’s a 20-minutes, once-or-twice-a-day thing. If I’m right, then this VR revolution is going to have some major bumps in the road.”