*ahem* I'm not sure if feature and gimmick are the same ? But i'll give a shot.
I just start to create a fangame with friends some days ago, so i have nothing to be proud of for the moment.
First, the gimmick of "I Wanna Be the white dot" is really interesting. Using two differents hitboxes according of the spike's color. Since i'm still consider myself as a newcomer, i discover some hard screens then i manage to pass them using this gimmick. It was enjoyable.
Second is "I Wanna Be The catharsis". I didn't play it yet, just watched Thenader2 speedrans it on hard and normal mode. But the artistic work is unique and AWESOME !!! It gives a special atmosphere that i enjoyed watching this game.
Third, a special graphic at one screen of "I Wanna Conquer the blow game 3" that i really like it, it's about the fake 3D, when you have to climb a tower, the kid doesn't move, but the tower move itself by rotating. This is so interesting and i didn't see this in other fangames.
Note: A third feature has been added