It's approachable in three different senses.
It's very easy to understand.
When you climb in, you're immediately greeted by a very familiar cockpit that promises a completely visceral experience. Cruising at highway speeds, the road doesn't feel that close until you look to your side and realize that your butt's just a few inches from the pavement. There's no door, and it seems like you could simply reach out and brush the highway with your fingertips.
It's not carved from a single block of unobtanium.
Even if you tick every option, it'll only set you back $24,000. For that you get several places to stash your gear, seats that are completely waterproof, a radio, a backup camera, and bluetooth integration. Forget the much more expensive Morgan; for some people, this is a viable alternative to buying a Miata as a second car to use on the weekends.
It's very easy to drive fast.
It's so well balanced and mild mannered that anyone, almost regardless of experience, can approach 90% of the Slingshot's capabilities very easily, unlike the vast majority of performance vehicles that require years of driver training before you can extract a car's true potential.