I've seen this, unfortunately sometimes in my church's worship service.
I think a lot of times, especially when introducing new songs, we focus so much on getting the music just right that we are not worshiping from the heart, which is key to leading our brothers and sisters into the Holy of Holies, so to speak.
The second biggest problem IMHO is a lack of dynamics. You can't start all fast songs at the volume ceiling or you can never take off. Of course, we all know consistent tempo is key to dynamics.
For me, if I were to describe not taking off, I'd say it's when the worship leader is unable to unify the praises of the people, and stay there long enough for the glory of God to descend upon the people.
Reasons for this happening include songs that are too complex, too many gaps in the singing, poor choice of keys, distracting music... and many of the other stuff that comes out in the pet peeves thread. :)
I guess my definition of a "take off" is like entering in as a whole congregation into God's presence during worship. It may be more of a spiritual thing than just a good sounding song or worship team.