A word for "interesting feature"

I am searching for a good english word that would be an analogy for russian word izuminka, direct translation is raisin, but it is used in figurative sense.
Other synonyms I've found: appeal, picanthy, zest.

Most close meaning would be an interesting feature.
So it is something that makes the object not so common and this single feature tends to add an improvement, mostly when speaking about visual appeal or taste.

E.g.: "She is not exactly an attractive woman, but she has an ... "

Also it should be possible to use the same word in ironical sense, so if put in quotes it becomes a mockery:

For example a new website or software feature:

A: What is this new annoying feature?
B: You don't get it, this is a "..."

Or nitpicking about someone's look:

A: Why does she wear these ugly yellow shoes?
B: It is sort of her "..."

Solution 1:

je ne sais quoi, defined by The Cambridge English Dictionary as

French for 'I don't know what': a pleasing quality that cannot be exactly named or described:

Although he's not conventionally attractive, he has a certain je ne sais quoi that makes him popular with the ladies.

This term fits the OP's three examples very well.

Complimentary:

She is not exactly an attractive woman, but she has a certain je ne sais quoi.

Ironic and mocking:

A: What is this new annoying feature?

B: You don't get it, this is a "je ne sais quoi"!

Nitpicking (with a few additions to B's response):

A: Why does she wear these ugly yellow shoes?

B: (with a shrug): It is sort of her...aah..."je ne sais quoi".

True, it is a French phrase, but widely used in English, and in an English dictionary.

Solution 2:

The rather vague term, something is often used that way

A remarkable or important thing or person: He thinks he is something in that uniform.

American Heritage

In your case, you could say

She is not exactly an attractive woman, but she has something.

It is often said with a slightly rising inflection that suggests the feature is difficult to quite grasp or express. It is related to the French phrase (often used in English), je ne sais quoi.

A related term is a certain something, as in

She is not exactly an attractive woman, but she has a certain something.

Related Phrases are something else and really (quite) something

Oxford Dictionaries Online

It also can be used in a slightly derrogatory manner, ususally connoted by tone.

He is really something. [The emphasis would be on really, drawn out with an ascending tone].

See, also, the lyrics of the Beatles tune, Something