In D&D in its purest form, what is the difference in play experience between a low- and high-level character?
There are a lot of play-experience differences that I don’t think are essential. For instance, in most editions, high-level combat has a tendency towards each round taking longer, each character having way more options, and more HP-ignoring attacks being made. But I’d be happy without those characteristics.
What else is there? High-level characters are more important in the story of the world? They have to deal with less logistical inconveniences?
Here’s an interesting statement from the 2e Dungeon Master’s Guide, in the section devoted to timekeeping:
At low levels, characters tend to go on short adventures. A few hours in the dungeon followed by a speedy return is about all they can survive. Therefore, it is easy to have a week’s interval within adventures, since the time passed does not impact on the characters’ activities. As characters reach higher levels, however, their ambitions grow and their adventures become longer.
This is a kind of interesting distinction between low- and high-level characters. In most editions, as characters level, each fight takes up a smaller and smaller proportion of the characters’ total HP. Therefore, higher-level characters can make it farther and farther into the wilderness before they need to return to home base.
As a level-based playstyle distinction, I don’t hate it.
It doesn’t 100% work, of course. At any level, a difficult fight might wipe out your HP and spells. Furthermore, characters quickly become self-sufficient. Characters who can generally recover their HP and spells in the wilderness have an effectively infinite range. Still, from level 1 to around level 5, there’s a nice increase in adventure scope.
If you wanted rules that pushed this further, what could you do? You might allow each character consecutive “roughing-it” nights equal to their level before they face difficulties from exposure and exhaustion. Thus, a first-level fighter can spend one night in a spooky haunted house, and then he needs to return to town. A thirtieth-level paladin can spend a full month in the Nine Hells, fighting to exhaustion every day, before she needs to return to the Prime Material Plane.
Alternatively, you can let economics take care of PCs’ increasing range. Besides being tougher, high-level characters are richer, and can afford horses and ships. If you’re faster than anything on the random encounter table, you can travel with ease.