Pretty much what @mesch said. While bridging amps isn't necesarily bad in and of itself usually, it can be if the speaker load is unstable or hard to drive.
I had a pair of Hafler DH-220s that I ran in bridged mode to drive a pair of ESL speakers. Sure, great power but something sounded "off". When I decided to use just one channel of each amplifier for each channel the difference was quite audible! There was much more detail and clarity and dare I say even warmer sound.
Yes having extra power is nice (440 wpc bridged vs 120wpc non-bridged) but depending on the speaker it can be problematic. Of course buying speakers that are efficient and easy to drive helps a lot. 
Happy listening.