An autistic savant can learn extremely well given precise parameters and boundaries...and yet may never learn to tie his shoes or look both ways before crossing the street. But we still call them intelligent.

I think that gets to the core question of intelligence. I think that is merely a question of functional complexity. I am more intelligent than an ant because the mechanics of my decision-making process are far more complex and tuned. The question of "Is X more intelligent then Y?" is meaningless without narrowly defining the word intelligence.

Practically speaking, intelligence is just the ability to achieve your goals. An AI, ideally, would have some kind of utility function and make predictions about what actions will lead to the highest utility. The better the AI, the better it's ability to make predictions and get the outcomes it wants.

The amount of complexity is irrelevant. There are terribly complicated algorithms which do worse than simple ones.

I think part of it would be formulating goals. If you have a process that can very rapidly sum numbers, it ranks lower than one that is incapable of that speed, but can decide that summing numbers is a means to accomplish that goal.