I'll keep it short:
If you've ever tried to play a new ladder season and found that after the first game, you can't find anyone or any game that's at the same stage as you are... you'll know how frustrating it is to try to play ladder. Unless you're lucky enough to have friends who play.
I'm sure we've all joined a game called "A1 normal" and found that they're in Act 3. Lots of stuff like that, adding up to not being able to find people who're at the same stage of the game, so you end up playing alone and wondering why you don't just play SP.
So, a suggestion is that this could be helped by having guilds, which allows people to schedule times to play together with some degree of trust in the people you're playing with.
If it's too difficult to do in the game code, maybe it could be done as part of the web site functionality.
I'd be interested to hear people's thoughts and ideas on this.