Mercury, brainy planet of thinking processes, communications, and the senses, is not often thought of as the most important planet in a natal chart. But it is.
Elsewhere I've referred to speedy Mercury as the 'tofu planet' since it tends to soak up the flavors of whatever planets it contacts in a chart.
Take Mercury conjunct Pluto, for instance. George W. Bush as a 1st house (Self) conjunction in Leo of Mercury and Pluto, a pair which together can indicate mental obsessions, cruel speech, and spying tendencies. One of the obsessions is, I suspect, tremendous mental focus (Mercury) on acquiring wealth and possessions (Pluto) and Pluto is god of the Underworld, thus he doesn't care how he attains his desires - or whom it beggars.
Well, W certainly demonstrated such things while he played president, but lack of intelligence wasn't actually one of his shortcomings in spite of how he was marketed and critiqued. Deep (Pluto) thinking (Mercury) is one of the signatures of Mercury/Pluto along with propaganda meant to sway the thinking of others; the conj posited in his natal 1st house shows that Mercury/Pluto was he.
On a lighter mercurial note, expert astrologer Beth Turnage says that Mercury is important for making love last in relationships, so check out Beth's excellent article, for if you've ever had a relationship where communication or its lack caused major problems, you already know the importance of mercurial compatibility in relationships.
And as they say, sexual attraction begins in the brain!