Start with the usual suspects at 7 months out: AKV concierge is almost never available, BLT standard is a close second to that as most often not available (and that remains true despite that more standard rooms were added effective Jan 2013), AKV value is next but does have its times when there is room such as in September, first half of Feb, and some other times. Next on the often not available list is BWV standard and boardwalk views, but you can find those at a number of times between mid-January and end of Sep. Also very difficult most times of year are BWV GVs; OKW GVs for some reason seem to be gone a lot in summer but open Jan through May. BCV is actually often open between mid-Jan and October but can be gone at times during then and is usually gone the rest of the year. Exceptions to any of the above are sometimes 1BRs which are usually the last to fill.
Other than the above, everything is usually open mid-Jan to just before end of Sep and that includes the holidays during that time, MLK, Presidents week, Easter time, Memorial Day and July 4, and includes GVs at AKV and BLT. Mid-Aug to just before end of Sep (including time around Labor Day) seems to be
DVC's slowest time of the year where you can even get lucky at times and find AKV value and BLT standard.
SSR is always open all year long. Moreover, statement above about difficulty of treehouses is no longer true. DVD did a point shift that raised the points needed for treehouses significantly for every night of the year beginning with reservations starting January 2013 with the result that the treehouses have thus far been wide open in 2013 at 7 months out through early July 2013 (the current 7 month window) and I suspect that pattern will continue.
That brings us to the quarter of death, last few days of Sep through the Monday after Marathon weekend in Jan, the time of high competition and where being on-line to make a reservation at exactly 8 a.m. eastern 7 months out can often mean the difference between getting a reservation you want and not. You have several factors at work then: (a) the lowest and second lowest point cost seasons during that time except for Christmas week and a few days around Thanksgiving; (b) food and wine festival late Sep to almost mid-Nov, and Mickey's Halloween party late Sep through Oct; (c) ideal weather for much of the time until Dec during which it can still be fairly pleasant; (d) Christmas decorations beginning before Thanksgiving; (e) often low air fares in first half of Dec; (f) a lot of DVC members who are now beyond the kids-in-school stage.
The result is that getting any of the close to park resorts, BWV, BCV, BLT, and VWL becomes quite difficult. There are respites during that quarter where those are available, the third week of Dec and about Nov 15 to Tues before Thanksgiving. Nevertheless, you can still usually get AKV and OKW during October, November, and early January except for usual suspects noted above. Also, you can sometimes get lucky and get BWV pool/garden view and BLT lake view particularly if looking for only three or four nights in a row.
Nevertheless, within the quarter of death, there is: (a) the first week of Dec, the highest demand week of the year, when at 7 months even OKW and AKV savanna and standard view are difficult (the last AKV savanna view 2BR disappeared at 8:05 a.m. when the 7 month window opened) and it was the only week of the year where SSR GVs were gone multiple days; (b) Christmas week, the second highest demand week of the year, but you could still get OKW studios through 2BRs and AKV standard and savanna view for the entire week; (c) second week of December, the third highest demand week, which was close to same as Christmas week but if you acted quickly once the 7 month window opened on-line, you could get BLT lake view or BWV pool/garden.