[R. Scott]

A mortal sin is a sin so grievous that the sinner is no longer in a state of grace, e.g.: a man commits murder and dies without repentance, he will spend eternity in Hell.

First, I am not Catholic, but I see the difference between mortal and venal sin as one of intent:
The classical definition of mortal sin is given by St. Thomas: "When the will sets itself upon something that is of its nature incompatible with the charity that orients man toward his ultimate end, then the sin is mortal by its very object...whether it contradicts the love of God, such as blasphemy or perjury, or the love of neighbor, such as homicide or adultery."
Even more obtuse is the definition given by St. Augustine (Contra Faustum, XXII, xxvii) “as something said, done or desired contrary to the eternal law, or a thought, word, or deed contrary to the eternal law.”
The Catechism of the Catholic Church, paragraph 1857 has “For a sin to be mortal, three conditions must together be met: "Mortal sin is sin whose object is grave matter and which is also committed with full knowledge and deliberate consent."
In modern parlance, if a sin is committed with full knowledge that it is a sin it is mortal. If a sin is committed through blind passion or accident it is venal. Even a lie can be a mortal sin if it is told under oath.
From the Catholic Encyclopedia, “Hence the wickedness of invoking the Divine testimony to confirm an untruth is specially criminal. Prescinding from cases of ignorance or insufficient deliberation this sin is reputed to be always mortal. When in doubt one cannot without perjury swear to a thing as certain. When mental reservation is permissible it is lawful to corroborate one's utterance by an oath, if there be an adequate cause.”
With such a definition we have a whole lot of people living in mortal sin. If the Church’s rules are followed to the letter, there would be few people at the communion rail.

[GrannyML]

It is a matter not only of intent, but it must be grave as well. That is, if you tell your wife that her awful new dress looks lovely, that is a venial sin, while lying under oath is a mortal sin.

[sinkspur]

With such a definition we have a whole lot of people living in mortal sin. If the Church’s rules are followed to the letter, there would be few people at the communion rail.

Actually, with such a definition, there may not be as many people living in mortal sin as you think. Full, deliberate consent, grave matter, and full knowledge. Mortal sin separates one from God essentially; it ruptures the relationship.

Think of something that you do that can rupture the relationship between you and another person. There are things (such as losing one's temper or negligence) that can damage it, but it takes almost a premeditated act to kill a relationship with someone.

The present state, where Catholics think nothing is a mortal sin, is a reaction to years and years of Catholics thinking everything was a mortal sin. Catechesis is needed to bring our understanding back into balance.

[SaltyJoe]

"If the Church’s rules are followed to the letter, there would be few people at the communion rail."

The rules are not intended to deny Grace, but to discipline us to receive more. If followed "to the letter", there would be more receiving Communion because:

1 we'd would go to Church quite regularly
2 we'd go to Reconciliation quite regularly
3 we'd live in a State of Grace to receive Communion
4 we'd have more babies
5 we'd have more priests because families would have more babies
6 the more priests we have, the more available the Sacraments

Since many are not following the rules, most if not all are suffering by receiving less than what could have been if we had allowed the Holy Spirit to work through us more.