Do you get the same boost to friendship for mailing gifts to your visitors regardless of what the gift is? Do you get the same bonus for giving them a seashell or fruit as for expensive furnitue? Also, do they put in their house or wear what you give them, and if so what are the odds of them doing so?
In past games a shell (or a fruit) would do just fine. However the content of the letter also matters. In Wild World, you could write "Friend, friend, friend, friend" etc. and they would love it! However if they didn't understand the context, they would write back a 'not so nice' letter saying they couldn't understand you.
I don't know if they improved the villager AI in this one.
I'm not sure exactly what determines what they do or don't understand. Punctuation is important and there was a limited 'vocabulary' they could understand. Stationary was just for show and didn't make any difference.