Although historians dispute this issue to some extent, I would argue that Gorbachev's attempts at reform played an important role in the collapse of the USSR. Specifically, these reforms loosened Soviet controls enough that people demanded more and more changes until the system fell apart.
Before Gorbachev, there was little real hope of change in the Soviet Union. There were dissidents, but they tended to be suppressed and had no hope of truly getting the system to change. But then Gorbachev came in and argued that fundamental change had to occur. Once he admitted this, it was hard to draw the line. He tried to reform the system without destroying communist control, but his reforms opened the floodgates. The people were "given an inch" and they "took a mile."