We love WDW in Jan. Coming from a year round warm climate, we look forward to cool weather, a fire in the hotel lobby, hot drinks at night, and wearing a jacket or sweatshirt. We check the weather a week before we leave and plan for temps just a little cooler or warmer. For us, anything below 60 is chilly- if it is 50, we are cold! I was wearing gloves, knit hat, scarf and jacket one trip (and glad for them) but others were still in shorts. You might be one of those in shorts, laughing at me bundled up like I was in Alaska. (Or, you might be envying my warm hat and gloves!) We use layers- t-shirt, long sleeve shirt, and a light jacket. I like those soft ones that are warm but weigh next to nothing in a backpack.
With the arrival of large tour groups from SA, the parks are more crowded than winters years ago, but it is not like summer or spring break. The most popular rides will have waits of 30-60 mins, but many will stay around 15 or less.