Sign of the Sun is a mostly typical adventure game in that you pick up objects and use them in appropriate places. The difference is that similar to Omikron or Alone in the Dark, action plays a big role in the game: you will have to punch and kick your way through many portions of the game. Unlike those two games, the action interface in Sign of the Sun is badly designed to say the least: you are forced to use only the numeric keypad to kick or punch, and despite many tries I find it very awkward to use. Worse still, one of the game's most annoying action sequences takes place right at the beginning, thereby giving a poor first impression to the gamer.
If you are patient enough to get through the first few awful minutes, though, the game improves considerably until it gets worse again. You can use the mouse for normal adventuring portions - left click to interact, and right click to bring up short descriptions. Unfortunately, the puzzles are not terribly interesting to solve - think of the puzzles in Indiana Jones: Fate of Atlantis, minus the cool backstory behind any of them.
Die-hard adventure gamers will likely find Sign of the Sun merely an average adventure game that could do much more with the fascinating plot. Recommended only as a "novelty" kind of game - and if you simply must play every adventure game ever made.