Really, it speaks volumes about Katsulas that he took a character who only appeared four times (well, really, three. His cameo in All Good Things was just for laughs) and established such a sinister and memorable presence.
If I recall correctly also, the cast and writers weren't too crazy about the Romulans anyway, at least in terms of major recurring villains. They did in a pinch, but certainly not to the degree of development as, say, the Klingons or Vulcans.
The acting with Tomalak was good, but the focus was not there. There were hints of Tomalak being an interesting character, but it never really went anywhere, that was a shame.