If you use OpenOffices Formula-Feature, you can choose three alignments: left, centered, right. But if you chose left, not only the text ist aligned left but also the numerator of fractions ist aligned left.

This is a little example. If you use longer equations it looks even worse:
aligned to left


centered

I have often to write chemical formulas, e.g.:

NH_4 Cl + H_2 O "<->" HCl + OH^"-" + NH_4 ^"+"

this arrow ("<->") should be like in this wiki-article:
Gleichgewichtspfeil

It would be extremly useful to implement this, because rightarrow or dlrarrow are not the same

edit: I don't want to replace any arrow. I want to add this arrow - and its not the same as the arrow for resonance (I'm sorry, couldn't find it in the english wiki):
"Dies wird durch den Mesomeriepfeil (Resonanzpfeil) ↔ symbolisiert, der nicht mit dem ein chemisches Gleichgewicht symbolisierenden Doppelpfeil \;\leftrightharpoons\; verwechselt werden darf." http://de.wikipedia.org/wiki/Mesomerie

means: the arrow for resonance may not be mixed up with the arrow for chemical equilibrium