Canberra is definitely very pretty and relaxed, but I guess maybe it is just the size thing. Some people can't handle living in such a small city with a limited amount of things to do, and expect it to be more lively because it is a capital city and therefore seems like it should be livelier.
While this is all based on hearsay, it's a LOT of hearsay from a rather varied group of people. Even my friend's dad moved to Canberra and struggled to find things to do within his interests. One of my friends who is very involved in the Student Union in Melbourne dropped out of Law/Arts at ANU after 6 weeks to do Arts at Melbourne because she said the Student Union wasn't very active and there was little to no student life. But that being said, she enjoys a good drink, and apparently ANU don't allow alcohol at any student related events. Maybe that's the entire issue.
And people I know who've lived there have said police have seen them with marijuana and it's no issue. They probably exaggerate their experiences though. It sounds quite relaxed.