What are the most common sewing machine features I should look for?
Tension control, buttonhole feature, stitch functions and the type of bobbin the machine takes should all be considered when selecting a sewing machine...
Some models have "automatic tension control". Automatic tension control really only applies to the top thread on the sewing machine. The bobbin thread tension is still manual on all machines. Some form of automatic tension control is a benefit as it eliminates another part of the process.
Most models have a buttonhole feature but all are not the same. Some require more work than others. There are sewing machines that offer you the ability to create a buttonhole in one easy step while others require a little more work. If the buttonhole feature is important to you look for models that do it easily.
The number of stitch functions
In general - the more the better. There are computerized models that have more than 1,000 stitch functions. It is unlikely that you will need 1,000 but having to many is better than having to few.
The type of bobbin
Bobbin location usually falls into one of two categories. Front loading bobbins and drop in bobbins. In general, drop in bobbins are easier to use but there are many different designs available that has made front loading tolerable.