[ttfkam]

I have never heard of Castilian Spanish referred to as "accentless" before.

The lisp always sounds odd to me. That and vosotros. The vast majority of Spanish speakers worldwide use neither.

[fcatalan]

Of course it's just another variety, but it's the one equivalent to the Received Pronunciation or "BBC English". People also tend to consider it the "original" one because, well... the language started in Castile. Up until a couple decades ago, if you wanted to have a career on national TV, radio or as an actor, toning down any other accent and switching to Castilian was a requirement, or at least would help a lot. Nowadays not so much, but still happens enough that in Spain a Castilian accent is perceived as "not having one".

[salomon812]

Northern New Mexico uses "vosotros." No one told us not to. (Or so I'm told, see my main comment.)

[prpl]

I actually learned both in high school (Las Cruces) but moved to Utah and dropped vos/vosotros. (grew up in Farmington before that)

[anthk]

Not a lisp but proper differentiation. It really helps to avoid mistakes:

za, ce, ci, zo, zu -> tha, the, thi, tho, thu. As in 'think'. That's it.

[mejutoco]

Spanish/español is also called castellano. That gives a clue as to what the standard is (not that it is better or worse than any other dialect)