Ad Verbum was one of the first IF games I played, and still a favorite. In this mid-length, story-lite game, you must collect and throw away various items by using constrained commands (commands that don't use certain letters, commands that only use the top row of the keyboard, repetitive commands, etc.)
The most enjoyable rooms are the rooms where you must use only words starting with a certain letter. The hardest part is trying to leave! How do you exit a room to the north if you have to start all of your commands with s?
The top few levels and the backyard are not quite as fun. And there are some puzzles that are just weird "guess which random object has the random property you need" puzzles.