Only joining one is a little tight I think, if you have different groups of friends. Maybe you can only join 3 guilds max. That way you have your 1 major guild you're most active in, plus 2 on the side that's just for when the main isn't active, fooling around, etc.

Also, it'd be amazing to have games or some sort of special feature about guild houses instead of them basically doing the same job as a regular house. :o