What are the important differences between Canadian and American (USA) English?

Spelling

Canadian English tends to combine aspects of American and British spelling. Here are some highlights:

Some nouns take -ice/-ence while matching verbs take -ise/ense. eg. practise / practice and license / licence
Canadians tend to use the British -our ending rather than -or in some words like colour, flavour, labour, neighbour.
Generally, words with Greek roots end in -ize while those with Latin roots end in -ise. eg. realize, paralyze. American English tends to standardize on -ize.
You draw money from the bank with a cheque not a check.
French derived words like theatre and centre tend to retain the -re ending. Although when used as a verb or in the sense of being "in the middle" it remains center.

Vocabulary

Canadian English uses generally the same vocabulary as American. There are a few regionalisms and quirks.

A multi-level parking facility is a parkade.
Poutine is delicious.
In winter, you want to wear a toque(pronounced tu-que) to keep your head warm.
You wear runners on your feet, not sneakers.
Generally, you go to the washroom or bathroom (even when the referenced room does not contain a bath and is not intended for bathing) when nature calls. That term's generally been replaced by restroom in America and it's a public toilet or lavatory in Great Britain. In Canada, toilet is somewhat indelicate and avoided.
A pond in farmland is a slough. (Rhymes with brew.)
You book off work to go on holidays.
My favourite from where I live: it's not a hooded sweatshirt, it's a bunny-hug.

Pronunciation

Pronunciation has American and British influence. There is some regional variation, and for some words, Canadians vary between British and American patterns. There are many little quirks, here's a few:

The last letter of the alphabet is zed.
Borg are Canadian. Futile, fertile, fragile etc. usually rhyme with "tile".
Adult, composite -- accent is on the first syllable.
Roof and hoof rhyme with "goof".

What sets Canadian pronunciation apart the most is the phenomonon known as Canadian Rising. Certain diphthongs are "raised" before voiceless consonants (eg. f,k,p,s,t). While most Americans discern no difference between writer and rider, in Canada, the vowel sounds are distinctly different.

Canadian Rising causes the illusion of about sounding like "aboot" to American ears.

This is a huge question. Canadian English has many differences from American English. But it also has many differences from British English.

Spelling tends to favour the British way, such as putting the U in favour.
Except for words that Americans end in -ize instead of ise; in that case Canadians often use -ize.
Much of the word choice is closer to American. We say truck and elevator, not lorry or lift.
Canadians drink a can of pop, not a can of soda.
The last letter of the alphabet is Zed, not Zee.

In general where there is a difference between American and British you can guess that half the time Canadian goes one way and half the other. Occasionally we go our own way.

It is so tempting to say