Acts 2 we find that when those 120 people waited for the Holy Spirit, they did not know how long they would have to wait, because Jesus never told them. If they knew it would be only ten days, it would have been easier for them to wait. When God keeps us waiting for something, He does not usually tell us how long we have to wait for an answer. God wants us to live by faith, because that is the only way to grow spiritually - and waiting is one way by which our faith is strengthened. If we knew how long we had to wait, then there would be no faith there. It is only afterwards when we look back that we can say, "I had to wait for three days (or three years)" for that.
If you had gone to those waiting disciples and asked them, "How will you know when you have been baptised in the Holy Spirit?" - They would not have said that they would speak in tongues. They would have said that Jesus had told them that they would receive power. You may ask "How will I know that I have received this power?" God can assure us of that, just like He assured us that our sins were forgiven. The Holy Spirit, Who bears witness with our spirit that our sins have been forgiven will also bear witness that we have been endued with power. Ask God to give you the assurance for both these important matters. So they were waiting for power. But when they got power they also received the gift of speaking in unknown languages (tongues). The tragedy today is that many who claim to have the gift of tongues do not seem to have any power. Tongues can be imitated, whereas power to overcome sin and to be a bold witness for Christ cannot be imitated.
Consider this illustration: You go to a shop to buy a computer (power). The shopkeeper tells you that a free CD (tongues) is also being offered with every computer. You didn't go to the shop to buy a CD but a computer. But since the CD is being offered freely, you take both the computer and the CD. But now your friend sees your CD and goes to the same shop and pays for the computer but brings home only the free CD!! How foolish! That is a picture of those who received the gift of tongues but not the power.
We read that a noise and a wind filled the house where they were praying and tongues as of fire rested on everyone's head. These were only the externals. The receiving of an anointing and power were the major events. The externals were different when Jesus was anointed. In His case, there was no wind or fire but a dove and a voice. But the power and anointing He received were the same as these apostles received. We all need to be "anointed with the Holy Spirit and power" as Jesus was (Acts 10:38). The physical and emotional experiences may vary with each of us. If someone were to give you an expensive diamond as a gift, it wouldn't make any difference if it came wrapped crudely in ordinary brown paper or in a shiny paper with fancy ribbons? It is the gift inside that is important. Only babies are taken up with the wrappings, the externals - the wind, the fire, the noise, the tingling sensations, etc. Adults are taken up with the gift - the anointing and the power of the Holy Spirit. When people testify about and gory in the wrapping in which they got the gift, you know that they are still baby-Christians.
The tongues of fire that sat on everyone's heads indicated that the most important part of our body that God would be using in the new covenant age would be the tongue - a tongue set on fire by the Holy Spirit and under His total control all the time. This is also part of the symbolism of the gift of tongues. God wants to use your tongue to bless others with, not only if you are a preacher, but also in your ordinary conversations with people every day. But for this, you must let the Holy Spirit have full control of your speech, 24 hours a day, seven days a week. In 2:14, we see one wonderful result of the Spirit indwelling all the apostles. We read here, "Peter stood up, backed by the other eleven" (MSG). What Jesus could not accomplish in His entire lifetime is now accomplished at last: The twelve are all one body now. They did not compete to have the honour to preach. They supported Peter and were 100% behind him as he spoke. This is the greater work that Jesus had said they would do after the Spirit came - the Holy Spirit had baptised them into one body.
In 2:17, Peter said that the outpouring of the Holy Spirit was the fulfilment of the prophecy of Joel. All men and women could now have the Spirit poured out on them and they could prophesy. This was a privilege reserved only for kings and priests under the old covenant. But under the new covenant, all can receive the Spirit. We need to recognise what a tremendous honour this is.