First, it ended organizing on the grand, 1930s scale. It outlawed mass picketing, secondary strikes of neutral employers, sit downs: in short, everything [Congress of Industrial Organizations founder John L.] Lewis did in the 1930s.
The second effect of Taft-Hartley was subtler and slower-working. It was to hold up any new organizing at all, even on a quiet, low-key scale. For example, Taft-Hartley ended "card checks." … Taft-Hartley required hearings, campaign periods, secret-ballot elections, and sometimes more hearings, before a union could be officially recognized.
It also allowed and even encouraged employers to threaten workers who want to organize. Employers could hold "captive meetings," bring workers into the office and chew them out for thinking about the Union.
And Taft-Hartley led to the "union-busting" that started in the late 1960s and continues today. It started when a new "profession" of labor consultants began to convince employers that they could violate the [pro-labor 1935] Wagner Act, fire workers at will, fire them deliberately for exercising their legal rights, and nothing would happen. The Wagner Act had never had any real sanctions.
[…]
So why hadn't employers been violating the Wagner Act all along? Well, at first, in the 1930s and 1940s, they tried, and they got riots in the streets: mass picketing, secondary strikes, etc. But after Taft-Hartley, unions couldn't retaliate like this, or they would end up with penalty fines and jail sentences.
[31]
http://en.wikipedia.org/wiki/Labor_unions_in_the_United_States#Labor_legislatio n