As Christians we are all called to be the strongest Christians we can be, including being prepared to tell others about our faith. However, we are not all called for the same functions in the church--some are called to be preachers, some are called to sing in the choir, some are called to teach Sunday School (okay, I'm taking a rather modern license to Acts 6:4, 1 Cor 12), but the same spirit holds true of "one body, many parts."
As far as ordination, this is how I understand it: Clergy should be called by God to be ministers, since only God can supply the talents necessary for effective preaching. If someone has been "set aside by God" to be a minister, then they should do these two things: Make sure their own lives live up to the moral standards they are to preach (1 Peter 5:1, Titus 1:5) and make sure they have a thorough understanding of Scripture. Pastors are (literally) called to be shepherds leading their flocks. This is a unique position within the church, and the laying on of hands signifies this. I'm not sure how all Protestants feel in regard to this rite, but this is my own personal take on this ceremony: I don't think the laying on of hands actually passes on a spritual authority or changes that person making them more "worthy" to preach, but is rather an external reminder of the special task that God has given this person. Like the sacraments of baptism and the Eucharist (and I include prayer as a sacrament), it is a physical reminder of internal grace. This physical reminder is important for two primary reasons, including: 1) It shows that this person has the blessing of other church leaders to hold this position (so, unlike what happened to poor Paul at times, there is no questioning of his/her authority as a spiritual leader) 2) It is a personal reminder to the minister that s/he has been set apart by God to live a life that should be above suspicion.
Thus, although we are all priests (in the Old Testament sense of being allowed continual access to God and offer Him our sins for forgiveness) and we should all live holy and faithful lives accordingly, not all of us are called to be pastors of congregations. In my opinion, the role of pastor includes 1) Preaching to already baptized Christians to remind them of their faith, 2) Baptizing and offering the congregation the Eucharist, 3) Counseling their Christian members in all matters concerning their faith, 4) Sharing the message of Christianity with non-Christians (the laity are also called to do this in their daily lives, so the functions overlap here).
You also asked, what denoms require ordination? As far as I know all mainstream Protestant denominations, including Baptists, require some form of ordination or licensing. This is their way of ensuring some degree of uniformity in preaching standards (like seminary training) and also for the reasons stated above.