Taking the last point one step further, we can also dispense with the if block entirely:

    if [ a = b ]; then
        echo "Oops!"
    else
        echo "Expected; phew!"
    fi

becomes

    [ a = b ] && echo "Oops!" || echo "Expected; phew!"

I'm not sure how often you should do this but sometimes it comes in handy for things like

    [ "$debug" ] && echo "what's going on" >&2

to conditionally print debug output to stderr.

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And the fact that the if block tests a regular command means we can also do things like

    if grep -q 'debug' /var/log/nginx/access.log; then
        echo "Debug request found!"
    fi

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Something I have not yet bothered to figure out is whether I should write

    [ $(expr 1 + 1) -eq 2 ] && [ $(expr 2 + 2) -eq 3 ]

or use the built in logical and of test:

    [ $(expr 1 + 1) -eq 2 -a $(expr 2 + 2) -eq 4 ]

As long as performance is not a concern, I can see roughly equal reasons in favour of either.