Actually, PM I found Total Depravity to be the linchpin, not Limited Atonement. In fact there are many Calvinists such as Dr. Steve who do not hold to a strict adherence to Limited Atonement. There is much debate on this within the Calvinists circles but I never found it to be much of an issue.
But as for Total Depravity I find the Calvinists are ALL in agreement. Everyone else (RCCers, Orthodox, and non-Calvinist Protestants) are on the other side of the fence. Total Depravity is where you'll find the argument on "free will" at. No one wants to admit that man is truely corrupted and cannot help himself except for the Calvinist. It is through an understanding of Total Depravity that one can understand Irresistible Grace and Perseverance of the Saints. Unconditional Election is just an extension of Total Depravity. Since we are unable to make the choice God chooses. It's really NOT Unconditional Election you disagree with or fail to understand. It's Total Depravity. This is the lynchpin of Calvinism and its what separates us from Arminian thinking.
For the most part, I would agree that Total Depravity is the most important point, However, Edwin Palmer stated that if any point of TULIP were wrong, then they are all wrong.
Also, P-Marlowe did not say the 'L' was THElynch pin, he said it is A lynch pin.