When a person watches a '50s drive-in film with a script written over the weekend, directed by a first time director, and starring the director's girlfriend who can't act, they cut it some slack.
When a person watches a modern, $100 million dollar film with a script written and rewritten by 5 people (but still reads as if it was written over the weekend), directed by an award-winning director (who can barely keep the camera straight), and it stars well know actors (who, for some reason overact or sleepwalk through it), they aren't so forgiving.
For me, a low-budget movie has to be obscenely terrible for me to completely hate it (Howling: New Moon Rising, Ancient Evil: Scream Of The Mummy). I expect more of a film which actually has talent and money behind it.
To put it bluntly, there's no reason that Batman & Robin should have been what it was. Nothing works in it. The acting is on par with a high school play, the script is mind-numbingly bad, there are simple editing mistakes, and the director could barely keep the shots in frame. There was a lot of talent and money involved (the writers later won an Oscar for writing A Beautiful Mind, for God's sake) and, yes, it is one of the worst films of all time.