I have played all sorts fmultiplayer and single player rom Frontline Force to They Hunger to ETC to Azure Sheep to OpFor to Ground Zero and many more
The ones that do it for me have a challenge in skill and thinking... some hard bits, easy bits, blast 'em bits and some nice scenery...
Its the ambience that keeps me coming back to particular maps and mods...
Inspiration for my maps and tinkering come from all things.. life, tv, movies, cartoons, books, other maps... imagination is the key... think about something and then try and use the limited tools to make it happen.. thats the way to learn to map.