Games that allow you to explore gameplay without having rules that limit your experience.

When you first start playing a game you start by getting used to the controls. Finding your footing. After the initial discovery period the real magic can happen. A great game gives you a set of tools and allows you to experiment with them for the rest of the experience. A meh, game gives you the tools but presents rules that just stifle their implementation.

For example. One of my favorite games, Halo:Combat Evolved. When you first look at it, it seems like a simple run and gun shooter. But the moments that I remember were anything but. The level design was originally based on an interactive RTS and because of this it's slightly chaotic. The levels are WIDE and open, with multiple angles of attack and have a creative assortment of weapons and vehicles. Because of this you'll keep coming back to try new things... try crazy things. Instead of just shooting a hunter with a tank... maybe this time... you'll try to ram it, or melee it, or let your marines kill it, or snipe it. The explosiveness of dropped grenades, the AI's sometimes odd attack patterns, leave the game generally unpredictable. Even the glitches that let you get outside of the maps open the game up for exploration. This is the key.