Counter-Strike has changed very little over the past decade, and aside from new maps and a couple of new modes, Counter-Strike: Global Offensive [official site] has mostly stuck to old routines. Thank goodness for modders, then, who add things such as maps which randomly select weather patterns at the start of each round.
So far they've released versions of Cache, Dust2, Mirage and Overpass which select randomly from ten available weathers at the start of a round. That means you might be playing in heavy rain one moment, and a sandstorm in the next, and then heavy fog after that. The above linked Steam page has details of all the available weathers and what features they include, such as thunderstorms which adds heavy rain, low visibility, puddles, lightning and storm clouds.
"When raining it is harder to both hear enemies approaching and to see without the aid of a sniper rifle," co-creator Luke Millanta told Eurogamer. Millanta had the idea in August and worked with another modder, Lewis Palfrey, to make it happen. "This means that while your enemy may have an advantage while AWPing [aka while using the Arctic Warfare Magnum sniper rifle, famous for one-hit kills], it is easier for those stealthy players to sneak up on said AWPers."
I was always fascinated by the roads not taken during Counter-Strike's early development. The dabbling with vehicles, the abandoned VIP assassination modes, the gimmick-led maps such as de_railroad. They all made for a more varied game, all were rejected by the community, and the result has been relative conservatism in new iterations like Source and Global Offensive. I hope the popularity of mods like the above gives Valve license to be more bold in how they develop the (still great) game.