Sharpness or crispness almost never has anything to do with the quality of the camera. And the D5100 is fully capable of outperforming essentially all professional cameras of less than 15 years ago.
Even if the settings are "identical" they may not mean the same thing. Moreover, there are so many settings that it's hard to imagine how they'd be identical. For example, the D5100 has probably twice as many settings as the D50 does, just for starters. And the Canon has different terminology and some quite different types of settings - so it's hard to imagine how a XSi can be made "identical" to a D5100 in setting configuration.
I don't know the details of the D5100 (or D50 for that matter) but I know that setting sharpening to either "auto" or "default" means very different things on several of my cameras, and I'd guess that the D50 and D5100 are similarly not equivalent.
Finally, most complaints about lack of sharpness or lack of crispness are due to - frankly - operator error or misunderstanding, not poor quality. Even the least capable or least impressive modern cameras and lenses are capable of really crisp, vivid results.
So... the best thing to do is as mentioned a couple of times above: let's get a representative sample, along with basic information such as lens, focal length, aperture, shutter speed and ISO - and then we can try to figure out what went awry. (All of that information is stored in the file itself, by the way.)
_____ Brian... a bicoastal Nikonian and Team Member
My gallery is online. Comments and critique welcomed any time!