I would find automatic dimension conversion useful.  I frequently find myself looking at, e.g., http://www.wolframalpha.com/input/?i=2+cups+flour to convert US volume measures to grammes.  I do not own US measuring cups, and find it more convenient to weigh ingredients rather than squint at a measuring jug.

I realise, however, that this requires a database of ingredient densities, so is much harder than a simple unit conversion.

(In reply to Matthias Clasen from comment #0)
> Not sure if more detailed preferences are necessary.

Sadly, more detailed preferences might be useful, since "standard" units like tablespoon and cup vary across countries, even ignoring imperial/metric. Australia is probably the worst example of this; we have a standard 20mL (instead of 15mL) metric tablespoon measure, which is a huge difference in something like a baking recipe.

Imperial and US Customary units differ too.  The fluid ounce is slightly different and the gallon has a different number of ounces in each system (and all units that are related to the gallon).

Even though the Imperial system has been officially eliminated in most countries, it will live on in recipes.

Doing a direct conversion of a recipe written in one unit to another unit will likely produce unusual measurements, such as "4.058 tsp" of something.

If someone ends up cooking the recipe with minor changes to the ingredients, such as snapping all the measurements to the nearest easily measured amount (for instance, "4 tsp"), perhaps Recipes could provide a convenient way to contribute a "validated" variant, and show that instead of an auto-converted version when people request a conversion.  An auto-converted version could show an appropriate note somewhere saying that the recipe was auto-converted from $unit_system, and a suggestion to write down and contribute a variant with converted measurements.

A similar system of "variants" could also allow for substitutions, such as for people who modify a recipe for their dietary requirements.

I would definitely like to see conversion to weights from volumes, as well as conversion from metric to imperial etc. A preference to show units using non-abbreviated forms would be nice as well, it took me a while to realize that "dl" stood for deciliter - a unit that's infrequently used in the US even when we do use metric unit.

Also, from looking at some of the recipes included so far I think that it would be useful to have some indication (esp when creating a recipe) that certain measurements are ambiguous, especially the "pkg" unit and that the ingredient description should more precisely specify the size of the package.  This is important because package sizes can vary based on a number of factors like geographical region, brand, time (it's not uncommon for manufacturers to change package sizes to maintain profit margins without changing the price of the product), "family" or "value" sized packages, etc.

this is an outreachy project idea: https://wiki.gnome.org/Apps/Recipes/Contributing/Units

I've merged this work now