One of the combinations I've missed since becoming dairy-free is grated cheese on baked potatoes. Is there any good substitution for this topping that would provide the salty-tangy counterpoint to the potatoes that I'm looking for?
We've used Daiya dairy-free cheese to make pizza and it's pretty close to the real thing. http://us.daiyafoods.com/
Since I've gone (mostly) dairy free, I tend to use salt plus a drizzle of a good olive oil and maybe some herbs.
For the you could use a bit of sumac, or za'atar blend (which contains sumac).
Another good sour note is tamarind, but I'm not sure how best to get it onto the potato. (you could use worcestershire sauce for the salt & tang, which is a sort of mellow tamarind-y fish sauce)
A dairy-free (tofu-based) sour cream or cream cheese imparts both creamy texture and a bit of tang. They aren't quite as tangy as the "real" versions, but come very close, particularly when mixed with other ingredients such as a potato. The main downside is that it can be somewhat harder to find either of these in a small grocery store.
If those aren't available, then olive oil, salt, and garlic powder is my favorite dairy-free baked potato topping, which gives a good balance of salty, fat (helps bind the potato together), and tang from the garlic.
Adding nutritional yeast to the garlic-oil potato tastes a little more cheese-like, but it generally needs to be well-mixed (I don't like the flakes on their own, just blended with other things for flavor) and also gets back to the possibly-hard-to-find problem :)
I'm assuming the non-dairy requirement is due to something particular to milk products and not because you are vegan. So I'll count egg as non-dairy in this answer.
On that basis, mayonnaise or salad cream make a good topping for the potato. Some sharp sauces such as HP sauce, Worcester sauce or hot chili sauce work well too. More regular meat sauces such as curry sauce, chili sauce, black bean sauce or sweat and sour sauce will work.
I think mayonnaise will be the closest you can get to the cheese feeling and taste and will beat any tofu based cheese substitute. I personally like the baked potato with mayonnaise and a sprinkle of bacon pieces.
If you are vegan, mayonnaise can become Vegenaise or similar.
you can always try a dairy free cheese which is created through using plant based milks, specifically nut milk based cheeses are very flavoursome.