“A pervasive developmental disorder, Asperger syndrome is distinguished by a pattern of symptoms rather than a single symptom. It is characterized by qualitative impairment in social interaction, by stereotyped and restricted patterns of behavior, activities and interests, and by no clinically significant delay in cognitive development or general delay in language.[15] Intense preoccupation with a narrow subject, one-sided verbosity, restricted prosody, and physical clumsiness are typical of the condition, but are not required for diagnosis.[5]”
http://en.wikipedia.org/wiki/Asperger_syndrome
To better understand the difference between a diagnosis of Aspergers and other syndromes, the DSM-IV TR would be the manual of choice. All DSM diagnoses of psychiatric disorders involve a specific pattern of symptoms. As our knowledge expands, these patterns and diagnoses change.
That said, the Wikipedia definition makes it clear that Aspergers is quite a bit different (and far less severe) than MR or autism. Perhaps, that is one reason it is over-diagnosed (along with AD/HD) in educational settings.