The game has a random level generator to create a near infinite supply of levels, and the built in level solver makes sure that every level is beatable.
You can change the window dimensions arbitrarily after the game has started which is great if you're playing a game on the side of doing something else. The game board also resizes to fit the height of the window. Only problem is that some HUD elements don't re-position based on the width.
In games like minesweeper, sometimes you run into a 50/50 chance situation where you have no choice but to guess at the solution. The generator in Hexcells runs every puzzle through a solver to make sure that all puzzles can be solved with logic alone without any guessing or brute forcing a solution.
The music in Hexcells is slow and relaxing for a great zen puzzle session so you can zone out if you're on a break or traveling. The sound effects also procedurally match with the music in tempo, so they aren't distracting, and they actually add to the music.
The clues provided in Hexcells come in a variety of forms. They can be based around number of active cells around an inactive one, the row, or the number of active cells up to two spaces away from an active cell, and all these have potential modifiers on top of it. The game introduces you to the concepts slowly so it isn't too overwhelming, but all the different possible mechanics help keep the puzzles fresh and interesting, and makes you think in more ways than games like minesweeper will.
While the gameplay starts out as being quite straightforward (therefore easing the player into the game), the difficulty does build as one continues to play.
The game can be beaten in about six and a half hours, which is really not all that long for a PC game. Of course originally this was a mobile phone game, which shows its roots in the length.
The game starts out pretty easy and gradually introduces new elements into the gameplay in order to get the player used to all aspects of the game as later levels can be pretty involved.
If you're into mechanical machines and clocks, you'll like the puzzle aspect of Cogs. The goal of the game is to construct a series of gears in 3D space to rotate a golden gear.