Currently, when defining a font feature one has to enable all features
by hand which is IMHO not very user friendly as it implies prior
knowledge about OpenType font features and the meaning of each one, not
every Arabic user, for example, knows what does 'init', 'medi, etc.
ligatures mean yet to know that he must enable them to get proper font
rendering. 

I think some font features should be on by default, so that
\definefontfeature[script=arabic] should be enough to get an Arabic font
rendered correctly with the default features as its designer intended
(designers assume that certain will be on while other are off by
default, like liga vs. dlig), and if some one wants to disable a certain
default feature he can turn it off, not the reverse.

Microsoft's OpenType features list page
(http://www.microsoft.com/typography/otspec/features_ae.htm) gives a "UI
suggestion" for each feature noting if it should be on by default, I
think those are what most OpenType enable by default (at least the ones
I tested).

Regards,
 Khaled

-- 
 Khaled Hosny
 Arabic localizer and member of Arabeyes.org team