The interface. All that grid of available actions, just too many options. Later quests evolved to have fewer actions (see Full Throttle), and later the only option left was general 'interact' action.
In this regard Loom was already ahead, with all actions replaced by general 'cast' action, and the exact combination of notes was the puzzle.
Reminds me of the sarcastic lick and smell actions they added to Space Quest when they were pressured to move away from the old text based input system.
They were the most memorable features of the game they were in.
It is easy to understand, why. Graphical adventure games were direct successors of text adventure games, where players interacted with the world via text commands, and you could have dozens of them without cluttering the interface.
It's not my dream per se, but in Monkey Island 1&2, when stuck and without an internet walkthrough, it was extremely frustrating to try out all the actions on all the objects. Full Throttle and Grim Fandango improved on that old model immensely, and Loom's interface turned out to be 10 years ahead of the curve.
I think that type of thing can really enhance a game if done right. It engages more of your senses and gets you (literally) thinking outside the box. It makes you feel like you're learning and demonstrating a skill, but in actuality the skill is just being attentive. The original came with a booklet [1] where you could write down the spells next to explanations of what they did, seems like an excellent way to do it.
Btw when I played it for the first time I just didn't understand that the pattern played backwards did the opposite. I thought that those are different spells, and it made things so much harder. Only many years later when I came back to replay Loom I noticed that some patters are suspiciously similar... oh boy.
Been probably 30 years since I played it last, but that would so make sense wouldn't it. I don't think I figured that out back then, wow. Still, the interface compared to all the other adventure games at that time was just so different and full of wonder, I kept coming back until I finished the darn game.
In this regard Loom was already ahead, with all actions replaced by general 'cast' action, and the exact combination of notes was the puzzle.