Can't agreed more with this. My previous iPhone 5 has much stronger vibration, and I can easily recognise even a short vibration while it's on my desk.
The problem seems to be that iPhone 6 vibration, while vibrates pretty strong when you hold it, is very silent when it's put on a desk. It might have something to do with the vibration direction. I think the vibration in iPhone 6 is moving horizontally instead of vertically when laid down, so it doesn't bounce the phone up and down, hence very silent vibration.
What we could expect right now is probably a 3rd party vibration motor with stronger vibration intensity, or a different orientation of vibration.
There are lots of vibration motor part for iPhone 6 on ebay. Some of them have completely different looks compared to the original ones, not really sure to try replacing with them except they advertised with stronger vibration though!