Many, many other things. Here's ADK's checklist for a winter day trip, and for a winter section hike. The student handbook has a lot more details.
You can see the size of packs that my buddy Chris and I were carrying for a winter day trip. Yes, a day trip. Oh, by the way, it's hard to see in the picture, but this is about a four- or five-foot snowpack. Up above, the wind blasted the ridge clear and it was all ice. We switched to crampons on the ledge in the next picture.
Falling off this, or getting pinned down by weather up here, would really have been dangerous. (It's been about five years since a hiker last died on that particular mountain, but it happens. Here we're getting ready to change out to ice axes and crampons for the final push to the summit.
You need that level of preparation - including the level of training provided by ADK Winter School, or the similar programs that AMC and a few other clibs offer - to be safe on the Northeast 4000ers in winter. A few hikers die there every winter. Do not be one of them.
Incidentally, Katahdin doesn't close - but support to AT hikers ceases no later than October 15 and weather-related closings become much more likely. The mountain is open for winter climbing. It used to be that the rangers would want to see your climbing résumé and check your gear before issuing a permit. I don't know the current rules.