One of the things I've observed is something of a mental block is strum timing. That is to say that people seem to think that strumming is like a militaristic 1-2-3-4-1-2-3-4-1-2-3-4-1-2-3-4 and so on.

Any rhythm pattern whether a rock riff or a strummed acoustic piece has a swing to it. A groove. Listen to an acoustic song and you'll realize that while there is a strum pattern and done so 'in time', it's ultimately a groove. A pattern that makes you want to bob your head or sway to it or some other thing.

The point here is to not so much listen for the timing but listen for the groove. What you'll notice is that there are little things with within the upstrokes and downstrokes that drive the groove.

For instance, George Harrison's 'My Sweet Lord' sounds like a solid 'Down-Down-Down-Down' (1-2-3-4) pattern but between the 3 and the 4 is a quick upstroke - D--D--D-U-D. That is part of the groove or the movement of the pattern.

Tricks like foot tapping or a metronome are timing mechisms and definarely have a use but when you think in terms of timing, it's often overlooked that it's really falling in to the rhythmic groove that people are looking for. Those things that make you bob your body. Those accents are what drive the timing. There are pattern changes and pauses that are really what makes the groove and accordingly, the timing.

Ultimately, that's the aim anyway. Groove.