According to Defenders showrunner Marco Ramirez, one of the goals of the project was to ensure that you could, indeed, watch the show without needing to watch all of the others (which amounts to some 65-ish hours of television):
“We had to make a show that was for the viewer who had only seen Daredevil. We had to make a show for the viewer who had only seen Jessica Jones, who had only seen Luke, who had only seen Iron Fist. And for the viewer where Defenders is the first thing they’re ever gonna see.
The interview is entirely focused on this question, and Ramirez consistently maintains that you don't need to have seen the prior shows to understand The Defenders, though obviously you'd miss out on some of the more impactful character moments.
You're inevitably going to lose something, partly because the show has four main characters and only eight episodes, and partly because it has to be engaging to viewers who have seen all of the backstory; however, as with The Avengers, the goal of the production was to ensure that you'd still get the essential experience.
Having seen the show1, I suspect you'd be mildly confused for the first three-and-a-half episodes or so. Until it gets around to having the four leads actually talk to each other, there's a tendency to introduce characters or concepts from prior shows without explanation2. Of course this isn't necessarily a bad thing, and you may find that it adds depth to the characters, but your mileage may vary. The central plot is timeless enough (evil ninjas want to destroy the city because reasons) that I don't think you'd have too much trouble picking it up.
That said, there are two elements that I think would be rather more jarring if you're coming in fresh:
- The relationships between Elektra, Stick, and Matt are quite directly continued from season 2 of Daredevil (and season 1, to a lesser extent). There is some exposition in the show to explain this, so I think you'd get the basic idea, but as with any emotional arc, the journey is more important than the destination
- If you have any prior familiarity with the MCU (especially pre-Doctor Strange), then you may find the injection of Eastern spiritualism and literal magic to be tonally odd. These elements are explored to some degree in Daredevil (particularly season 2), but are given fuller treatment in Iron Fist, while The Defenders just takes it as read that, yeah, qi is a thing
1 Though from a very different perspective, so take my opinion for what it's worth
2 And, though it's possible I just missed the explanation, I think the show flat-out forgets to define Jessica's powers, leading to something of an ex machina in the final episode