There is nothing special about Yorkdale anymore. It used to be THE place to go to check out some higher end stores - but those same stores have now opened up locations all over Toronto. The mall itself is fine, except for the terrible layout. This has turned into a big hangout, meaning there are never any free seats in the food court, and there is always some sort of incident involving security and/or the Toronto Police.
I do have to say, security at this mall is great. I had an issue myself, with a guy following me around with a very large knife. Before I knew it, security and the OPP had the guy in custody.
My advice? Don't go there to "hang out". Go if you'd like to check out Holts or some other semi-unique store.
If you dare to approach the food court - they have a Thai place that is great.