A slightly more rigid definition: "A spell is a mnemonic device to link the patterns and alterations of mana and a caster's understanding of the end result."

A comparison could be drawn towards performing arithmetic--with practice and experience some questions become rote, others are trivially broken down, and some still need actual methodology. Albeit magic is even less of a science than that due to the human element giving everyone a different mind and mana not being identical between people. Unique incantations can help with this, which is definitely a thing for experienced mages wanting to use full power for less cost. 

Inefficient casting is basically not fitting exactly right so you use more mana to bridge the power gap (or the result you're aiming for is so stupid). Catalysts, depending on preparation, can help by "filling in" part of the pattern, or just concentrating what they've got--so you can skip components or use less and get the same result.

As for a comparison with alchemy or enchanting: if all the power is derived from materials, the world, and procedure, then you're unlikely to need to vary things as much on a case-by-case basis (besides differences in what you're working with). If it's the sort of ritual which uses casters or directly changes human targets [i]then[/i] it encounters the same variability as full spellcasting.

Divine blessings are pretty much in the same boat IIRC because the gods are doing the complicated part there and the results are typically unreplicable or a pain to emulate.