Well, I'd say discussing dialects can be interesting, especially if a language has lots of different dialects (according to the official categorization). And like 邪悪歌 said, dialects can be quite different from each other in various terms. Dialects in English generally differ from one another in phonology and some vocabulary but I can understand most of them. An Australian from Sydney would still understand an American from New York pretty good, I think (this is only my assumption, I'm not a native English speaker

). In terms of grammar, English dialects still, for example, form plural by adding -s to a noun.
I can give you examples of Slovene dialects which differ from Standard Slovene in pretty much everything, morphology, syntax, phonology and vocabulary. I can actually understand Russian more than some of Slovene dialects, and I speak two dialects (out of more than 40 or 50 main dialectal groups).
Take for example the word
kolo /ko'lo/ 'bicycle' (Standard Slovene), in my native dialect we say
bičikleta /bitSi'kleta/ and in the other dialect I speak we say
kuolu or
kwolu /'kwOlu/. When it comes to declining this noun, the differences are even bigger.