Subforum would be optimal but splitting into Casual and Competitive with Competitive having subs for Standard, Modern... etc, would be the way to go imo.
I personally have 5 other casual decks I will posting up. Moving in January and that will put me 2 miles away from the shop instead of 25 so I plan on taking the decks up for some play.
Having them looked at by other eyes has already helped me tune the one I have posted.
As I've never taken then beyond the kitchen table, I appreciate the feedback.

I don't see how that's the problem when things are labeled properly. People deal with folders in folders in folders in folders every day on their computers.

No I don't. That is incredibly tedious, and at the very least, I can use the keyboard to navigate.

D:

I only have like 10 folders on my desktop and nothing else. Then everything is sorted and sorted more. OCD thing maybe.

I can see the benefit of having subforums. We essentially have the following options:

1) Make subforums. Nesting is annoying, but saves space and streamlines the forum visually and gives us a section for each format + casual, but at the expense of an extra click.
2) Make a ton of individual forums for each format.
3) Make a small split with only 2 or 3 subdivisions into subforums.
4) Make a small split with only 2 or 3 subdivisions into distinct forums within the magic section.

All of these have advantages and disadvantages. #2 is probably impossible for space concerns. #4 is probably possible now, but what happens when we have more posting and more formats start to warrant their own sections?

In Constructed there is a thread on the front page that hasn't been posted in since Wednesday.

Adding forums is completely unnecessary at this point.

I'm not so sure; a lot off the threads aren't just posting games that will get posted in several times a day. Some deck threads require several days between posts while people test, add/remove cards, etc., and a lot of times discussion is just so much chatter without that.

At the same time, when something falls off the first page, the chances of anyone new posting in it drop dramatically.

I'm not ruling it out, but I don't think post age alone is an argument against splitting a forum.

That's a valid point. I rarely go beyond the first page. Didn't even think of that.

At the moment, are constructed forums are formatted to where you just post a deck and get feedback and that's it. I don't think this is necessarily bad, but if we want to develop the constructed area into a place in which you are able to look up decklists in a given format and have resources in which people can understand what the top decks are in a format, etc. we will need to change the boards. I don't think at the moment we have enough posters interested in a single format to warrant splitting them into separate formats because that is what killed the wotc constructed section in the end, but splitting based on "seriousness" could be fine.

Then just make it competitive and casual.

EDIT:
Competitive as in they intend to bring it to a tournament.

Casual =/= bad.

That is the problem splitting it that way. I don't want people to feel marginalized by what section their deck is in or there to be animosity between the users of each section since I think there will be a divide.

If you're going to try splitting into groups of formats, while also trying to keep people from feeling marginalized, I think there are a few decent approaches (and probably several others, as well):

1) a) 1v1 and b) Multiplayer.
2) Chronological split: a) Vintage, Legacy-legal cards and b) Modern frames only
3) Mown's suggestion...with those awesome names.
4) a) Eternal and b) Rotating.

In all cases, there's probably going to be at least some amount of friendly competition, and people identifying with one section over others, but that may help keep down some of the feel-bad aspects of a Competitive/Casual split.