Sid was great until the bell rung. At that point, he no longer seemed all that scary. I'd say the same for Nash at his worst actually (He has periods where he was fine.) They don't need to be superworkers at that size, but being able to portray themselves as a threat through their matches in the same way they did leading up to them would have helped both guys out. But in Sid's case, he was so clumsy and couldn't even throw a forearm without botching it, and any feeling of this guy being a threat that was there before was suddenly gone. That's what people are getting at when they call him a bad worker -- not that he should have been able to do moonsaults and a bunch of suplexes, but that he should have been able to execute a few basic wrestling moves in decent fashion.
Sid had an unbelievable physical presence. His body, his face, his maneurisms were great. Watching Sid squashes in 1991 is pretty entertaining.
That being said, I agree with waht Loss said. The issue is that he was physically imposing, but he could only look devastating on offense if someone was making him look devastating. Otherwise, Sid could look like a clumsy goof. He was especially goofy on selling and bumping. That said, there has been times I enjoyed Sid. His super hot period in 1996/early 97 constitutes his peak to me, working with guys like Vader, Shawn or Bret surely helped things a lot. But Sid was all about what the other guy would do.
I feel Kevin Nash was better than that. More dynamic to his few moves I thought. It's no wonder Nash had much better matches than Sid had with the same opponents, and a variety of good matches against a variety of workers.