Signal to Noise ratio is just one aspect. You might have great SNR but if too many devices are trying to occupy the channel at the same time you get poor throughput. A good SNR is one that allows you to max out your router connection speed - and that varies by model of course.
Bad SNR is usually caused by interference - both in the sense of objects in the signal path and other transmitters. Placement and orientation of the antenna(s) can be critical and it's often trial and error that determines what's best. Wifi router power and channel settings are usually set at the factory to a "good enough" value and shouldn't be messed with unless you've got a really good reason to do so. And of course the SNR is dependent on the system you're using to get to the router. A bad antenna (or other problem) on the laptop / desktop can drop the SNR too.
Sometimes it's just a matter of expectations. Router manufacturers tend to overstate the throughput of their devices. They spec them at "IDEAL" conditions and of course your house or office isn't going to be IDEAL. You may need to adjust your expectations - which is usually a matter of asking others with similar hardware what they are experiencing. I have a wifi system with a 54 Mbit spec'd rating, but I'm satisfied with a real world value of about half of that in a three story house with thick walls.
