We took our first trip to Disney World in September and LOVE it, and want to go back soon! I'm considering another Disney World trip for my family, possibly October or December of this year... or somewhere around then.

What I'm wondering is... is it better to book ASAP, or to wait? Some things I want to take into consideration are things like:
Are plane tickets going to likely be cheaper farther from the date, or closer to? (I'm in Canada.)
I can't answer this one. Here in the US, plane ticket prices vary depending on time of day, time of year, rain or sun or clouds ...
Seriously though, I don't know if Canada has a set pricing schedule for air tix, but here in the US, we definitely don't. I just book when the price is something I can afford and hope I got the best deal.
Am I going to have a better chance of getting the resort I want (will be first time at a Disney Resort, as we stayed at the Hilton Downtown Disney last year) if I book sooner? Will I run the risk of not getting rooms later on?
If I wait, is it possible that promotions or room discounts will become available later on and closer to the date of the trip?
There really is no "slow season" at WDW anymore. September (after most kids go back to school) is typically a less crowded time of year to go, but with WDW offering various promos for that time of year, it has been getting busier and busier as the years go by. If you have a specific place you want to stay, and a specific room type, book it now. You can, in fact, book everything you would want now - ie: room, dining, and tickets if you're looking to book a package.
Even if you don't wait, if a promo comes out later on, you can have that promo or discount applied to your reservation. There won't always be a discount for the type of room you've reserved, however, so you need to decide if going up or down in room category - or even changing resorts - is worth getting the discount. Keep in mind, however, if you're already booked under a promotion, you can't "stack" discounts, so you'd have to figure out which one saves you the most money and go from there.
Is it good to wait until closer to decide which resort to book into, in order to review current client comments on places like Trip Advisor, etc?
There will always and forever be good and bad comments about every WDW resort. You can't please everybody all the time, and someone will always have something bad to say, while someone else who stayed at the same time will have a totally different perspective. If you have a WDW resort in mind you truly love and you're excited about the prospects of staying there, that's what matters - not what someone else's opinion is. Again, you and another family may be there the same week - you may have a fantastic time, they may not.
How soon do things like character breakfasts get booked up? (if we should decide to do any.)
Popular restaurants, such as character meals, Be Our Guest (BOG), 'Ohana, LeCellier, tend to get booked right at the 180 day mark. If you will be staying at a WDW resort, though, you get to book up to 10 nights of your stay right at the 180 day mark, which gives you a better chance of getting the reservations (ADRs) you want. Plan out your restaurants and decide which ones you really want, and book those as soon as you can.
Is there extreme weather in Orlando or things that can be predicted closer to the date that make it preferable to wait?
The weather in Orlando, much like anywhere else, is unpredictable. Hurricane season typically lasts through the end of November. It rains a lot in Florida. Sometimes it is oppressively hot and humid. If you go in October, the weather should still be pretty warm. If you go in December, I would advise you to take a couple pairs of jeans and a jacket. The key is to be prepared - and definitely bring ponchos or other rain gear!
Are there other things I should consider when deciding when to book sooner or later and what are they?
Have you looked at the various pricing "seasons" at WDW for the time periods you are considering? I don't know how important price is to you, but it can vary a lot depending upon when you're planning to go. I'll give you a link so you can look at the "rack rates" (pricing with no discount whatsoever) to give you a better idea of what I'm talking about. http://www.mousesavers.com/2013-walt-disney-world-resort-room-rates-season-dates/ That will help you better determine, should price be a factor for you, which time of year will be better to go. Of course, those are rates before any discount whatsoever is applied, so as long as you're okay with the base "rack rate" price, it can only (hopefully) get better from there.
So, how far in advance do you book your Disney World vacation, and why?
We have booked as much as a year in advance, and we've booked as little as two months in advance. For us it's easy because we have no children and can go any time of year (providing my husband can get the time off). We are pretty flexible and have stayed offsite more than we've stayed onsite. It seems to work out for us no matter when we've booked. If you really have your heart set on a specific resort/room type, though, I strongly recommend you book as soon as you can.
Thanks in advance!!