[phish]

The "key thing", as you call it changes as you progress as a player.
When you're first starting and playing against other newbies, you might have discovered that starting off with good hands makes all the difference in the world. (Today, with all these books teaching you these things, it's not such a eureka moment anymore)

As you develop, you may have found that playing a proper mathematical game is the key.

Then you discover that betting aggressive, especially against weak tight opposition, is "the key"

Now you're at the stage where you think proper 'semi-bluff' (only one of many strategy tools by the way) is the key.

Next you'll find that knowing your opponents' tendencies and exploiting them is the key.

Then you'll learn to target your opponents moods and what his image of you is.

Then you'll figure that being able to quit when you're losing and playing bad gives you the long-term edge against other great players who can't exercise such discipline.

Fact is, as you advance, the opposition becomes tougher. A "key" that beats one group of opponents will simply not be enough for the next group.