In my gaming history I met a few games that had mechanics rarely present in games of same genre. Let me start...
RTS: Remembers player's actions between missions in campaign.
In Red Alert 1, in allied mission 2 player have to build as usually and in mission 4 returns to that map, but extended, while player's units and buildings stay the way they were left at end of mission 2. Second occurrence I've only found in Warzone 2100, where main base is built and left way it was for duration of entire campaign, including modifications in-between missions.
RTS: Player's units travel for entire duration of campaign, if they survive mission. Any units built are added to travelling army.
This mechanic presents in LOTR: Battle for middle earth and in Warzone 2100.
And you, what mechanics have you encountered that can be added in any game of appropriate genre, but mostly stays either unpopular or rare?

