I compliment you and you crap on me and tell me I’m not helping my image. Thanks a lot. :mad:
I wish I may,
I wish I might
Have this wish
I wish tonight. 
Actually, as Omi no Kami says, in the field of linguistics there doesn’t really exist a distinction between “language” and “dialect”. So the question of whether Scots is a language in its own right or an incomprehensible dialect of English is mostly meaningless.
I’ve read a few – presumably non-linguistically accurate – definitions of what it means for two dialects to belong to the same language. One of them says that it requires that they share at least 80% of their vocabulary. That’s interesting, but at the same time, there’s a lot more than simply vocabulary that distinguishes languages from each other. Two dialects could have a similar vocabulary but so different accents that they are barely mutually intelligible. Another definition is just that: two languages are the same if they are mutually intelligible. But the fact is that mutual intelligibility isn’t transitive: if A and B can communicate with each other in their own language, and B and C also can, there’s no guarantee that A and C will be able to communicate with each other. So are A and B speaking the same language, and B and C speaking the same language, while A and C aren’t?
Consider the case of Hindi and Urdu, spoken in India and Pakistan, which I am given to understand are similar languages, though mutually unintelligible in their “pure” versions. Between the places where “pure” Hindi and “pure” Urdu are spoken, a whole spectrum of dialects going from one to the other is spoken. Also consider Afrikaans, which became a language when South Africa decided to make it one of its official languages instead of Dutch.
(By the way, if I’m saying something incorrect in this post, can one of the actual linguists correct me? I’m only an interested amateur.)
As for GuyNblueJeans’s (and others’) confusion, I think a lot of it stems from the fact that much effort has been done by prescriptive grammarians and teachers to stigmatize language that deviates from the prescriptive “norm” as “lazy”. Of course, being lazy is morally objectionable, so speaking lazily should also be, right? And on the surface it does seem that saying “libary” or “nucular”, for example, means replacing sounds that are “hard to say” with others that are easier, which means not having enough regard for the language to use it correctly, which means being lazy. But it’s not actually objectively true. It isn’t harder to say “nuclear” than to say “nucular”, but the second one contains sounds that are less unusual to speakers of some English dialects, which is why it is the pronounciation they use. My first language is French, and I’m sure it contains sounds that many of you wouldn’t be able to pronounce, while I have no trouble at all. But that doesn’t mean you’re lazier than I.
I think it’s been shown that as languages evolve, you can never say that they become “more complex” or “less complex”. As structures disappear (with people lamenting their disappearance), others appear.
So it isn’t lazy to speak the way you’ve been told not to speak because it’s “lazy speech”. But this doesn’t mean that people shouldn’t learn a normative dialect. For better or for worse, not a single dialect (or register) is appropriate for all situations. So the way you speak with your friends isn’t “wrong” or “lazy”, and you shouldn’t be ashamed of it, and maybe from it will evolve the normative dialect of a few centuries from now. But at the same time you can’t really use it for a job interview today if you want to impress people. That’s what being a naked ape is all about. 
Oh! Yea! I forgot all about mentioning that; I would hesitate to use hard numbers (x% of shared vocabulary), although some other linguists will disagree with me there. Either way, however, mutual comprehensibility is often a great informal test to figure out where the dialects end and the languages begin. (even this doesn’t always work though. Modern German speakers can pretty easily communicate with Yiddish speakers, for instance, if both parties are willing to put up with some awkwardness. Yet we can pretty easily demonstrate that even though modern German and Yiddish are in the same family, they’re decidedly different from each other.)
It isn’t like that at all. Hindi and Urdu are not two dialects of the same language - they are the same language. The grammar and base vocabulary (ordinary everyday words) are identical. When spoken the two are mutually intelligible.
The written forms use different alphabets and look as different as can be, but in the spoken language there’s no difference, apart from choice of vocabulary and a few details of pronunciation. But even then, these details overlap and there is no line that can be drawn to distinguish spoken Hindi from spoken Urdu. I really don’t think the word “pure” can have any meaning in this context.
The main difference in the literary languages, besides the alphabet, is that Urdu uses a learned vocabulary of Persian loanwords, while Hindi uses Sanskrit loanwords. If a Hindi speaker used a lot of Sanskrit vocabulary, it would be hard for an Urdu speaker to understand, but it would be the same as you or I listening to a scientist speaking English with lots of technical Greek and Latin vocabulary that we don’t know.
OK, I’ve checked a bit on the Web and it appears you are correct. I believe that what I remembered reading was this, as found in the Wikipedia article:
Which is basically what you said. Would it be correct to say that there exists a common subset of both Hindi and Urdu that can be used as a complete language, but both Hindi and Urdu also contain additional words that aren’t shared with the other?
Yes, exactly. Although I disagree with Wikipedia about them being two dialects. Literary Hindi and literary Urdu are the same dialect,* as used by two different speech communities. It’s like how slang or jargon (within a dialect) are used to identify the members of in-groups. These are whole languages, not slang or jargon, but they’re similar in that the purpose for their distinct existence is mainly semiotic, to mark out communal identity. For this you have to understand the history of communal politics in India.
*cite: Colin P. Masica, The Indo-Aryan Languages (Cambridge University Press, 1993).
The spoken base language they share in common is known as Hindustani or khari boli (‘upright speech’). This is no pidgin but a full language in itself. You can speak eloquently in basic khari boli and be understood equally well by Hindi and Urdu speakers.
In fact, that’s pretty much what Hindi movie dialogue consists of, as long as specifically Hindu or Muslim concepts aren’t in focus. The lyrics of Bollywood songs were for a long time conventionally based on Urdu models with lots of Persian vocabulary and verse forms - Urdu is felt to be the more poetic of the two, because the North Indian literary tradition since Mughal times had favored Urdu for poetry, developed according to classical Persian models.
It gets a little more complicated. The name “Hindustani” used to be applied to Urdu during the existence of the Mughal Empire, because the Perso-Arabic script was normally used by all authors of literary khari boli, Hindu and Muslim alike, in those days. It was only in the 19th century that the language became politicized and rather artificially divided into two things, one in Devanagari called “Hindi,” while “Urdu,” an alternative name for Hindustani, became elevated in order to emphasize the alleged difference. The purpose was to assert Hindu communal identity in the post-Mutiny period. See One Language, Two Scripts: The Hindi Movement in Nineteenth Century North India by Christopher R. King.
I’m not sure I understand what you mean–but I think I can guess. Do you mean the difference between the two is akin to the difference between the ways women and men talk in Japan? In Japanese, men and women have different personal pronouns, different flavoring particles, and other differences besides. But it would probably be wrong to say that they speak different dialects just because of this–rather, they speak one dialect, part of which includes rules about which kinds of people may use which kinds of constructions.
Is that what you mean?
-FrL-
Yes, that sort of thing.
All right then, thanks. I’ll never call Hindi and Urdu dialects again. 
-FrL-