As a professional numerics guy, I started getting interested in the underlying question here of "how close to D&D is game X". So I approached it as a machine learning problem.
First, define the dimensions of similarity. That's a hard start, but I decided to read Wikipedia's article on D&D and use the section on mechanics to come up with dimensions of similarity. Looking at the concepts that merited their own section, or the concepts that had significant descriptions I came up with ten dimensions. One (task resolution) was a multi-sentence description, so my restatement "Rolling d20+factors > DC to accomplish task" is more subjective than the others and therefore particularly open to disagreement or re-statement.
Next, come up with a way of ascribing similarity. I went with a 6-point scale:
5: It's so similar that I wouldn't note any differences to a D&D player
4: It's essentially the same, with some tweaks that would work in D&D easily
3: There's the same idea there, but the implementation is a bit different
2: That concept isn't there, but you can use another mechanism to do a similar thing, and that would be a normal thing to do
1: It's really not a thing in this game, but you could it if you wanted to, it would just be odd
0: There's no mechanical way to do this that doesn't seem incredibly forced
Finally, a way is needed to accumulate these scores into a single measure. For simplicity, I just went with adding up and taking the percentage of the maximum possible score (50).
So below is my table of how I perceive systems I have strong familiarity with to be similar to D&D. I'd be interested in seeing other people fill in their tables and see what the results are as the judging on the scale above is bound to be fairly subjective:
As expected
13th Age is VERY similar.
Savage Worlds and
BRP are somewhat similar.
Numenéra,
Gumshoe and
Fate not terribly similar. This bears out my experience of running campaigns using these systems for D&D players pretty well.