Two more things:

I get an error when trying to write outside current wd; modifying slightly the example in ?fwrite:

fwrite(data.table(first=c(1,2), second=c(NA, 'foo"bar')), "~/Desktop/table.csv")

Error in fwrite(data.table(first = c(1, 2), second = c(NA, "foo\"bar")), :
No such file or directory

Handling for list columns is probably incorrect. Not sure whether we should support... seems like it will be a good counterpart to sep2 in fread. As of now:

DT <- data.table(a = 1:3, l = list(list(1:6), list(3:5), list("a","b")))
fwrite(DT, "test.csv")

Produces:

"a","l"
1,list(1:6)
2,list(3:5)
3,list("a", "b")

I don't know the proper way to handle this, perhaps surround a list in angle brackets and comma-separate the elements?

"a","l"
1,<<1,2,3,4,5,6>>
2,<<3,4,5>>
3,<<"a">,<"b">>

Or perhaps take the cue from format.data.table:

format.item <- function(x) {
  if (is.atomic(x) || is.formula(x))
    paste(c(format(head(x, 6), justify = justify, ...),
            if (length(x) > 6) ""), collapse = ",")
  else paste("<", class(x)[1L], ">", sep = "")
}
if (is.list(col))
  col = sapply(col, format.item)
else col = format(char.trunc(col), justify = justify, ...)

Otherwise just error on list columns