Martin - I've had the same thought before. My idea was to have a "Hints" pop-up system that can be turned off in Options.

Brief Hints would pop-up anytime an action failed and also contain a link to the on-line manual section for more info:

Blend = Make sure you've selected edges, not curves. Make sure edges are not broken into segments.

Fillet = Make sure edges are joined. Try a smaller fillet value.

Planar = Make sure edges are planar.

Etc....

There are several things you just have to know to be proficient with MoI and I think a Hints option would reduce the learning curve. It would take some time to implement, but probably save more time by not answering as many forum questions.

Probably the forum regulars would be willing to write the hints as a group.

Ed