Poor maintenance: turning cozy fires into safety hazards
As time goes by, carbon begins to build up on the inside surface of the glass. This produces a whitish film that can obstruct the view of the fireplace. If neglected long enough the carbon can become ingrained into the glass and impossible to remove. Once you reach this point, your only option is to replace the glass to restore its original look.
However more importantly, like any gas appliance, there is a risk of carbon monoxide. A very dirty gas fireplace can choke the burner and cause it to produce CO. A block vent can also create CO and can remain unnoticed until identified by a technician; hopefully before it does any harm.