Personally I can think of a few (irrational?) fears:
Turing off your brain. At times trusting the Spirit comes across as not thinking. Too much critical thinking and logic can be seen as stifling the Spirit.
No discernment. Everything and anything can be written off as a leading for the Spirit. Since it is the leading of the Spirit it cannot be questioned. 98% of the time when people have told me God lead them to do something they are not asking for conversation and discernment. The voice of God has been used to kill discernment. A side note, it is strange how often God changes his mind. And that change of mind often involves the opposite gender.
These fears have caused me to limit my openness to the Spirit. But these are the easy fears to admit to. Chan raises two other fears that I would like to deny having (though that would be a lie):
What if God doesn't come through?
What happens if I ask for God to fill me or to lead me and nothing happens? It is easier and safer to hold back, to not expect God to show up in my life.
Do I even want this?
Do I really and truly want God's Spirit to lead me and guide me? Am I prepared to live that kind of life? Am I prepared to go anywhere and do anything for Jesus? Really, go anywhere and do anything.