Thanks for your thoughts, Paul.
Long-only traders (especially if trading on a longer timeframe than a single day) as well as investors tend to do particularly well in Dec for obvious reasons (outlined by you). Long/short traders may be at a disadvantage, however. For instance, if a long/short strategy does not adequately take into account market seasonality and market volume levels in its approach, then abnormally large and/or abnormally frequent losses in the strategy developing during a holiday period, especially on the short side, would not be surprising. Short trades that would be avoided by a more sophisticated (but not necessarily ultimately better) strategy can end up being taken by a less sophisticated (but not necessarily ultimately worse) strategy, during a holiday period.
Nov and Jan both normally have a reasonable amount of volume and shouldn't be considered holiday periods, in my opinion.
Taking profits quickly is a valid option, but then the stop used on trades would also need to be tighter than normal to compensate for this approach, I would think. Otherwise one losing trade in Dec could wipe out a number of other winning trades during the same month but that were only small winners.