Well, you'd have a larger Ainu minority overall, in addition to other minority groups like the Nivkh and Orok (And maybe a few more Japanese of Russian and Korean descent) having some presence there. I dispite General Tripitz's claims of a population celling of about 1.2 million, as the Island at it's height probably had around 750,000 people (Probably hit twice, in the early 40's and mid 70's), and the abundance of natural resources would probably end up attracting a disproportionate share of immigrants than you would expect by projecting OTL growth rates (especially if Japan avoids a conflict as nasty as WWII was). So I would say anywhere from 1.7-2.5 million being the potential range for an ATL Japanese Sakhalin.
Also, remember Sakhalin Island is large, about 27,000 sq mi (or twice the size of Kyushu and about 80% the size of Hokkaido). Though the climate means that it's growing season is rather short, Japanese Sakhalin would also probably contribute a considerable agricultural surplus.
In influencing Japanese Culture, we might see the frontier as a stronger theme in Japanese culture given that the process of expanding to new land would be taking place over a longer time, over a much larger area than OTL, and I imagine that Sakhalin (and the people from there) would acquire a sense of weirdness and exoticness due to it being really far from the metropole (Kind of the way most of America regards San Francisco OTL).
As for other butterflies, keeping all of Sakhalin might mean that Japan keeps the Kurile Islands and Taiwan to the present, which is a whole other can of worms.