From: http://sanctaliturgia.blogspot.com/2004/09/confiteor-deo.html

Why did the Church change the Confiteor? Few Catholics ever heard an explanation after Vatican II. The different versions and reasoning is given below:

Confiteor Deo

Missale Romanum 1962
P/S: Confíteor Deo omnipoténti, beátæ Maríæ semper Vírgini, beáto Michaéli Archángelo, beáto Joanni Baptístæ, sanctis Apóstolis Petro et Paulo, ómnibus Sanctis, et tibi, Pater: quia peccávi nimis cogitatióne, verbo et ópere: mea culpa, mea culpa, mea máxima culpa. Ideo precor beátam Maríam semper Vírginem, beátum Michaélem Archángelum, beátum Joánnem Baptístam, sanctos Apóstolos Petrum et Paulum, omnes Sanctos, et vobis, fratres (et te, Pater)1, oráre pro me ad Dóminum Deum nostrum.

S/P: Misereátur tui omnípotens Deus, et dimíssis peccátis tuis, perdúcat te (vos) ad vitam ætérnam.

P/S: Amen.

P. Indulgéntiam, absolutiónem, et remissiónem peccatórum nostrórum tríbuat nobis omnípotens et miséricors Dóminus.

S. Amen.

P/S: I confess to Almighty God, to blessed Mary ever Virgin, to blessed Michael the Archangel, to blessed John the Baptist, to the holy Apostles Peter and Paul, to all the Saints, and to you Father, that I have sinned exceedingly, in thought, word and deed: He through my fault, through my fault, through my most grievous fault. Therefore I beseech blessed Mary ever Virgin, blessed Michael the Archangel, blessed John the Baptist, the holy Apostles Peter and Paul, all the Saints, and you brethren (you, Father), to pray to the Lord our God for me.

S/P: May Almighty God have mercy upon you, forgive you your sins, and bring you to life everlasting.

P/S: Amen.

P.May the Almighty and merciful God grant us pardon, absolution, and remission of our sins.

S.Amen.

Missale Romanum 1970
All: Confiteor Deo omnipotens et vobis, fratres, quia peccavi nimis cogitatione, verbo, opere et omissione: mea culpa, mea culpa, mea maxima culpa. Ideo precor beatam Mariam semper Virginem, omnes Angelos et Sanctos, et vos, fratres, orare pro me ad Dominum Deum nostrum.

P: Misereatur nostri omnipotens Deus et, dimissis peccatis nostris, perducat nos ad vitam aeternam.

C: Amen.

All: I confess to almighty God, and to you, my brothers and sisters, that I have sinned through my own fault in my thoughts and in my words, in what I have done, and in what I have failed to do; and I ask blessed Mary, ever virgin, all the angels and saints, and you, my brothers and sisters, to pray for me to the Lord our God.

P: May almighty God have mercy on us, forgive us our sins, and bring us to everlasting life.

C: Amen.

The Confiteor has gone through periods of change and development. (See the Catholic Encyclopedia article at the NewAdvent website). The differences in the older form and the current are negligible. The revised form retains the confessional portion, and the request to the Angels and Saints and especially the Blessed Virgin Mary for intercession. The revised form of the Confiteor is actually more theologically sound in that it adds the sin of omission to our confession, because one sins also by not doing good. I also have a deep appreciation for the older missal and older prayers. I even say my Rosary in Latin (which by the way has been very helpful in moving beyond the words to a more contemplative prayer of the Mysteries). In no way do I wish to criticize or detract from the older missal, but rather to present a picture that objectively analyzes the differences while expressing the continuity of the two Roman Missals allowed for the celebration of the Holy Mass.

That being said, the two versions of the Confiteor in the Latin editions indeed do differ, but these differences are negligible in my opinion. Both versions include a statement of confession to God first of all, and then to the Church for our faults. In the older rendition certain saints were explicitly named that are omitted in the newer missal. This does not imply anything different theologically, because our confession is to God first, and then to the Mystical Body of Christ, the Holy Catholic Church. Who can object to the older use in that the Priest represented not only the alter Christi (to whom our confession has already been directed) and also as representative of Holy Mother Church and the local ecclesial community who also belong to the Mystical Body of Christ through baptism into the same. Who then can object to the revised version which makes confession also to God and to the local ecclesial community (which by default includes the Priest) there as members of the Mystical Body of Christ? The revised form of the prayer was indeed simplified in the first half, and happily the place of the Blessed Mother retained as well as a general request to the angels and saints for intercession on our part.

There can be no real objection to this change other than simply love for the older version. Otherwise you are in a position where you have to criticize the Rite of the Dominicans whose Confiteor names the Blessed Mother, St. Dominic, and the saints in general (omits mention of angels, but included by implication in the allocution of 'saint') in the confessional half but does not include any in the request for intercession!2 Indeed the locution of 'et omissione' (and failed to do: sin of omission) is found also in this Rite, most probably under the influence of the even older Gallican Rite. The same is true of the Carthusian Rite. According to their liturgical texts that predate Vatican II and the 1970 Missale Romanum.

It seems to be clear is that the change is more probably explained as a simplification of the prayer itself that relied on other forms than the Roman Missal. If some like the older version better, I don't blame them- it's beautiful and theologically sound. Though it cannot be denied that the addition of 'et omissione' is an improvement theologically as well as having a fine liturgical tradition in the Gallican Rite and in the Holy Monastic Orders of both Dominicans and Carthusians. You'll also notice that the 1970 Roman Missal made concessions in its rewriting of the Confiteor, neither simply replacing the traditional Roman version with Gallican or other those of other Rites, but instead blending the historical liturgical use of the Latin Rite traditions (hence retaining the mea culpa, mea culpa, mea maxima culpa, though the ICEL's translation makes this fact invisible).

It is not required that certain specific words be spoken before the approaching of the Altar of God, but rather that each individual prepare themselves to either celebrate or assist at the Holy Mass (whichever is proper to their role). Hence the laity enter the Church, genuflect to the tabernacle, and by immemorial custom kneel and pray to God as they prepare for the Holy Mass. The lack of preparation for serving at the Altar isn't really due to the newer missal but rather to a lack of catechesis.

1 The Priest says the Confiteor first, the Server replies with the Misereatur, the Priest replies Amen. Then the Server says the Confiteor, the Priest replies with the Misereatur, the Server replies Amen.
2 P: Confiteor Deo omnipotenti, et beatae Mariae semper virgine, et beato Dominico patri nostro, et omnibus sanctis, et vobis, fratres (S: et tibi pater), quia peccavi nimis, cogitatione, locutione, opere et omissione, mea culpa; precor vos orare pro me. See the Ordo Missae Dominicae