2 Nephi 9:30-40

30  But wo unto the rich, 

           who are rich as to the things of the world.  

       For because they are rich they despise the poor, 

            and they persecute the meek, 

            and their hearts are upon their treasures; 

      wherefore, their treasure is their god.  

      And behold, their treasure shall perish with them also.
31  And wo unto the deaf that will not hear; 

            for they shall perish.
32  Wo unto the blind that will not see;

            for they shall perish also.
33  Wo unto the uncircumcised of heart, 

           for a knowledge of their iniquities shall smite them at the last day.
34  Wo unto the liar, 

           for he shall be thrust down to hell.
35  Wo unto the murderer who deliberately killeth, 

           for he shall die.
36  Wo unto them who commit whoredoms,

           for they shall be thrust down to hell.
37  Yea, wo unto those that worship idols, 

           for the devil of all devils delighteth in them.
38  And, in fine, wo unto all those who die in their sins; 

           for they shall return to God, 

           and behold his face, 

           and remain in their sins.

2 Nephi 9:30-38 (Emphasis mine)

(Formatting for these verses comes from The Book of Mormon, A Reader's Edition, edited by Grant Hardy, pgs, 86-87)

Jacob lists a number of sins that we can commit as well as the consequences of these sins. He is making sure we fully understand the real consequences of making the wrong choices. 

As I've read this before, the phrase "uncircumcised of heart" is one with which I am not familiar.  I searched on the Internet and found an explanation I liked.  It is from Barnes Notes on the Bible.  

"Uncircumcised in heart - Circumcision was a sign of being a Jew - of acknowledging the authority of the laws of Moses. It was also emblematic of purity, and of submission to the Law of God. The expression "uncircumcised in heart" denotes those who were not willing to acknowledge that Law, and submit to it. They had hearts filled with vicious and unsubdued affections and desires."  (Emphasis mine) (source - Online Parallel Bible, Acts 7:51, accessed May 2, 2011.)

As we read the scriptures, we are told to constantly submit ourselves to Christ.  For me, this would be the foundation of the "wo's" from Jacob.  The root of all sin ultimately comes from our unwillingness to submit to the laws and commandments of God.  If we are, we will have help overcoming these sins.  We won't have to do it alone.

One of my heroes, the late Elder Neal Maxwell said in October 1995 conference:

"... the submission of one’s will is really the only uniquely personal thing we have to place on God’s altar. The many other things we 'give,' brothers and sisters, are actually the things He has already given or loaned to us. However, when you and I finally submit ourselves, by letting our individual wills be swallowed up in God’s will, then we are really giving something to Him! It is the only possession which is truly ours to give!" (Emphasis mine)

Sister Anne Pingree, 2nd Counselor in the Relief Society General Presidency said in October 2005:

"In even the smallest details of each day, I submitted my will to the Lord’s, for I so needed His help, His guidance, and His protection. As I did, gradually my relationship with my Father in Heaven changed—in profound ways—that continue to bless me and my family." (Emphasis mine - thanks also for her comments from Elder Maxwell that lead me to his talk.)

Being "uncircumcised of heart" takes us away from "... His help, His guidance, and His protection..."  It leaves us vulnerable to all of Jacob's "wo's" which ultimately will take us away from our goal of returning to Him.

39  O, my beloved brethren, remember the awfulness in transgressing against that Holy God, and also the awfulness of yielding to the enticings of that cunning one.  Remember, to be carnally–minded is death, and to be spiritually–minded is life eternal.
40  O, my beloved brethren, give ear to my words.  Remember the greatness of the Holy One of Israel.  Do not say that I have spoken hard things against you; for if ye do, ye will revile against the truth; for I have spoken the words of your Maker.  I know that the words of truth are hard against all uncleanness; but the righteous fear them not, for they love the truth and are not shaken.

2 Nephi 9:39-40 (Emphasis mine)

Jacob, using plain and clear language, tells us to give in to the passions and appetites of the flesh, the ultimate result is our death.  How much more clear can our warning be?

16  Know ye not, that to whom ye yield yourselves servants to obey, his servants ye are to whom ye obey; whether of sin unto death, or of obedience unto righteousness?

Romans 6:16 (Emphasis mine)

14  But every man is tempted, when he is drawn away of his own lust, and enticed.
15  Then when lust hath conceived, it bringeth forth sin: and sin, when it is finished, bringeth forth death.
16  Do not err, my beloved brethren.

James 1:14-16 (Emphasis mine)

We've got the world of the Lord as our guide.  He has given his scriptures.  He has given a Prophet, as well as inspired leaders to help guide us.  

It's really a no brainer, isn't it?