The biggest threat may come from the possibility of Democrats removing or altering one of the two controversial provisions in the bill in order to get Arlen Specter back on board. In case you're having trouble keeping track, Specter is now at risk of switching his stance on card check (again) and coming out in favor of the bill.
In order to make him more likely to do so, Democrats could alter the secret ballot provision enough to make it amenable to Specter -- meaning it would be slightly less repulsive to Republicans, but still repulsive nonetheless. Altering the secret ballot provision would mean that the other provision - mandatory binding arbitration - would remain fully in tact.
Mandatory binding arbitration is every bit as scary as the better-known card check issue. It would move negotiations between unions and corporations into the hands of an outside arbitrator, who is appointed by the government, and who is more likely than not sympathetic to Big Labor.
Even if this arbitrator is completely unbiased, mandatory binding arbitration removes the essential back-and-forth of two-party negotiations and puts the fate of companies and workers in the hands of someone who has no personal stake in the issues at hand.
The bill calls for mandatory binding arbitration to be installed after 90 days of failed two-party negotiations. With the likelihood of an outside, government-appointed arbitrator being sympathetic to union bosses, it's probable that unions would simply choose to wait out the 90-day period and resort to binding arbitration whenever a dispute arose.
This would mean that businesses would be hit with a double whammy of still-repulsive secret ballot regulations, and the death knell of binding arbitration. Sounds like a picnic.