I'm no expert, but based on my experiences as a native speaker of English and a second-language speaker of Spanish, it is my impression that the differences between Mexican Spanish and Peninsular Spanish are no greater than, and probably less than (at least in the case of educated speakers) than those between American and British English.
Brennus mentions the lisping pronunciation of "c" and "z" used in most of Spain, but this and the few other small differences in pronunciation are certainly less than the notable differences between British and American pronunciation. It's difficult to really do a direct comparison, though, because English dialects differ mainly in vowel pronunciation whereas Spanish dialects differ in consonant pronunciation. Also, I'm not sure where Brennus gets the idea that "rojo" isn't used for red in Mexico -- I've run into both "rojo" and "colorado" quite a bit in Mexican Spanish.
Brennus mentions the lisping pronunciation of "c" and "z" used in most of Spain, but this and the few other small differences in pronunciation are certainly less than the notable differences between British and American pronunciation. It's difficult to really do a direct comparison, though, because English dialects differ mainly in vowel pronunciation whereas Spanish dialects differ in consonant pronunciation. Also, I'm not sure where Brennus gets the idea that "rojo" isn't used for red in Mexico -- I've run into both "rojo" and "colorado" quite a bit in Mexican Spanish.