One of the jokes about older versions of D&D is that there was nothing more deadly to a wizard than a bag full of housecats. It's a double edged joke (made utterly unfunny in explaining) that highlighted both the fragility of wizards, who had trivially small numbers of hit points, and the problems with assigning even a small damage value (1-2 points) to something like a housecat.
It also interestingly showcases some of the dangers you need to be aware of if you want to have more fragile characters in your D&D or D&D-like game. This is a popular trick for changing the pacing and level of tension in a game - by making the heroes (and their opponents) more fragile, you greatly speed up fights, but also make them feel more intense because of the intense sense of threat. It can also have unintended consequences as players become more risk averse, which can really bog down play.
My own experience with this is deeply rooted in Rolemaster, which took a more complicated approach to resolving the same issue. Characters had plenty of hit points, sure, but the damage tables were full of bleeding wounds, broken bones and no shortage of gruesome, instant death. The net result was a strong sense that any fight could kill you if things just happened to go the wrong way.
While this might have been paralyzing, we actually treated it as very motivating for two very specific reasons. First, it forced us to pay a lot more attention to the situations we were in to try to leverage them to our advantage. This meant that being sneaky and smart paid off.
Second, and more important, it forced us to really think about our characters and how they tied into play. As players, we faced two apparently contradictory facts: adventuring was genuinely dangerous, but we wants our characters to adventure. Forcing ourselves to resolve that contradiction made for MUCH more interesting characters, since they needed to have motivations capable of overcoming the potential risks. By itself that was sufficient to keep the game from slowing down, but it had another benefit.
See, when you have a character whose motivations are clear and strong enough to you that they overcome a risk (which is also clear to you) then you have a character who is more likely to start engaging the game more fully. That is to say, when your motive is to adventure, then you will adventure. But when adventuring is a means to an end, you will start looking for other ways to pursue that end. If your real goal is revenge against your uncle, then you might delve dungeons for power and wealth to use against him, but you might also look to disrupt his mercantile arrangements or harm his allies. This is pure RP gold.
None of which is to say that high-lethality is the only way to get that kind of investment. It absolutely is not. But if you find yourself in a game where players seem locked in a kind of dungeon-centric tunnel vision, seeing no reason to engage outside of that context, then perhaps a little extra risk is exactly what you need.
[back] 1 - I want to contrast this with 1e D&D, specifically a game I'm currently involved in. In that game, we also spend a lot of time planning, but that planning primarily engages the system. That is, we think about what spells to cast and gear to use, because those can provide substantial bonuses. Terrain and situation are helpful, but secondary concerns. Which is to say, being smart pays off, but it doesn't make things much more fun.