The biggest concern that I hear from most ladies is a fluctuating, or 'low' libido.
Female libido is so misunderstood and undermined by years of being told that dudes are horny and chicks are romantic that it's hard to work out what's really going on.
Many women report that they have little to no interest in sex, but that when their partner initiates and they play along for a little while... they end up MAD HORNY AND SUPER IN TO THE WHOLE THING.
This is because the majority of women have a RESPONSIVE libido, rather than a SPONTANEOUS libido.
Responsive desire is when the motivation to fuck begins AFTER sexual stimuli has been presented and arousal triggered.
Spontaneous sexual desire is just what it sounds like, feeling aroused and wanting to fuck for no obvious reason.
The traditional model for the human sexual response cycle is basically:
DESIRE -> AROUSAL -> ORGASM -> RESOLUTION
And this can often be the case! However, spontaneous desire is thought of as more typically male and responsive more female. That is not to say that either gender exclusively falls under either category. In fact, it's often the case that females experience mainly spontaneous sexual desire at the beginning of a new relationship, after a lengthy separation from a partner, or in a more casual/one-night-stand situation.
It's important for women (and men in the same boat) to realise that if they aren't feeling randomly horny all the time, especially in a long term relationship, it DOESN'T mean they have 'lost' anything or have no desire. It's normal, it's typical, and you're still a human with a sex drive.