As noted in the comments, there are a lot of axes to compare role playing games on. Here are a few:
D&D tends to be pretty heavy in terms of rules complexity. There are a fair number of special cases, but a lot of them are variations on the basic concept of "roll a d20 and modify it with the appropriate stat." You can find more complex systems out there, but many systems will be simpler.
D&D has varied pretty wildly on this axis over the course of its lifetime. D&D 4e tends to be extremely abstract... The rules operate on their own, and it's up to the group to interpret them.
Focus (combat, social, etc.)
D&D tends to be extremely focused on what happens during combat. Fights make up a large portion of any given adventure. This is not to say that you can't use it for other things... But the bulk of the rules covers what happens during a fight, with only simplified systems for other aspects (social encounters, stealth, economics, and so on).
Latter editions of D&D tend to make it very difficult for player characters to die. Other games make combat extremely deadly.
Are characters built from a template with small customizations, or from scratch from a variety of interchangeable components? D&D is the stereotypical level-based system.
How well does the system handle unusual group sizes? How much time does it take to prepare an adventure? How quickly can you adjust to someone not making it to a session? In all of these particulars, D&D tends to be relatively rigid (3.x being the worst offender).
And many more...
That's just a few off the top of my head. You might browse the system-recommendation tag to get a feeling for the kind of things that people look for in RPGs.