[ryberg]

On the original question, the great thing about the lack of the use of vosotros here in Mexico is that you get to forget the second person (you) plural verb conjugations wholesale! Everything with ustedes matches with third person plural (they) forms, which you already know. And as a bonus, you also get to forget all about the relevant personal pronouns (os, vuestro/a), too, similarly just using the third person forms you already know and use (su). So it's really just remembering to just say ustedes and then everything falling out easily from that, not having to pull up a form you don't usually use. It would be harder to go from Mexican Spanish into the form spoken by Spaniards, because then you would in fact have to pull up verb forms and pronouns that you don't use very much. (I can't even do it anymore when talking to friends from Spain, even though I did learn those forms originally.)

On the name of the language point, you will in fact get some differing views. At different times, friends of mine from Spain have made statements that themselves are contradictory in this regard. For example, one asserted that what was spoken in Spain was no longer really Castilian, but rather Spanish, emphasing the name of the country in the name of the language, exactly as some people will tell you (in all seriousness, too) that what is spoken in American isn't English but American. It was this person's opinion that due to the name/country connection and a supposedly more conservative nature in the language as it was spoken in Mexico, it actually made more sense to say that what Mexicans speak is Castilian. (In this he evidently didn't see the irony of the regional name connection being applied to somewhere outside Spain.) On the other hand, one other friend insisted that only Spaniards spoke Castilian -- not Spanish -- and that Spanish (Español) was in fact properly applied to the language as spoken everywhere else but Spain, emphasizing that its origin was Spain but at the same time that it wasn't true to the original aspects of Castilian.

Of course, all this is socio-linguistic and political meandering, and linguists all know that whatever you call them and despite the really very small differences between them, it's just one language, just as English (which shows more dialectic variation throughout the world) is just one language. You get this kind of inconsistency all over the place. We say Chinese as if it's one language when in fact the country has regions with mutually unintelligible forms that are less like dialects of the same language than Spanish and Italian, which we always consider to be two different languages even though careful speakers of each (even I, not as a native!) can communicate with each other without changing them. (I have seen my wife do this on multiple occasions with Portuguese speakers, staying in Spanish, and both sides understanding just fine.) In fact there's a good line about this by an Austrian, I think, whose name I cannot remember, to the effect that a language is a dialect with an army and a navy.

Steve