My question is what is the significance of laying on hands; or what does the laying on of hands represent? On the Day of Pentecost (Acts 2) the Holy Spirit fell on all assembled without the laying on of hands.

The laying on of hands has multiple uses in the Bible. However, in a general way, it almost always portrays the bestowing of something from one to another. Jacob laid hands on Ephraim and Manasseh in order to bestow a blessing them (Genesis 48:14And Israel stretched out his right hand, and laid it upon Ephraim's head, who was the younger, and his left hand upon Manasseh's head, guiding his hands wittingly; for Manasseh was the firstborn.
See All...). On the day of Atonement, Aaron laid his hands on the live goat in order to put the transgressions of the children of Israel "on the head of the goat" (Numbers 16:21Separate yourselves from among this congregation, that I may consume them in a moment.
See All...). Moses laid hands on Joshua in order to bestow upon him the authority for leading the children of Israel (Numbers 27:22-23 [22] And Moses did as the LORD commanded him: and he took Joshua, and set him before Eleazar the priest, and before all the congregation: [23] And he laid his hands upon him, and gave him a charge, as the LORD commanded by the hand of Moses.
See All...). Jesus often laid hands on people when He healed them (Luke 4:40Now when the sun was setting, all they that had any sick with divers diseases brought them unto him; and he laid his hands on every one of them, and healed them.
See All...).

These are just a few examples of several dozen instances of the laying on of hands in the Bible. You give some good examples in your question from the book of Acts. The practice clearly gave weight to any time when some sort of blessing or authority was being granted to another. However, Jesus did not always lay hands on those He healed. As you point out, the Holy Ghost did not have to wait for the laying on of hands in order to fill or indwell believers (see also Acts 10:44-48 [44] While Peter yet spake these words, the Holy Ghost fell on all them which heard the word. [45] And they of the circumcision which believed were astonished, as many as came with Peter, because that on the Gentiles also was poured out the gift of the Holy Ghost. [46] For they heard them speak with tongues, and magnify God. Then answered Peter, [47] Can any man forbid water, that these should not be baptized, which have received the Holy Ghost as well as we? [48] And he commanded them to be baptized in the name of the Lord. Then prayed they him to tarry certain days.
See All...). The laying on of hands gave an external picture of the bestowing of any gift, but it was not required for God to act.

I think it is good to include the laying on of hands in an ordination service. Sometimes people lay hands on people in special cases of sickness as they pray for them. This is certainly alright, though it is not commanded for us today. We must understand that the laying on of hands was symbolic. It does not make something happen and it has no magical power. God can work with it or without it. However, it adds gravity to a serious act and graphically shows the giving and receiving of certain gifts.