Is WD-40 suitable for bicycle chains?
No. WD-40 is more solvent than lubricant. It means "water displacement". For that it might work. For bicycle chains it is not much better than any other volatile solvent such as water (yes, if you have ridden in rain you have noticed a wet chain doesn't squeak so water is lubricating it -- when the rain stops, the squeaking starts).
Would alternatives provide better chain longevity?
Yes. Use a thixotropic motorcycle chain lubricant in a spray can.
The spray can is agitated before application. The thixotropy of the lubricant makes it thin after agitation. Thus, when sprayed, the thin lubricant penetrates to the innards of the chain easily. When left to settle, it becomes thick, staying inside the chain. When you start to ride, the thixotropy makes it thin again, not resisting motion as much. When left to settle, it becomes thick again, staying inside the chain and preventing drops of lubricant on the floor.
Also remember to only lubricate a chain that squeaks and remove all external dirt you can easily remove from the chain. Don't lubricate a dirty chain! Don't also lubricate a chain that already has oil. If the chain has (clean) oil inside, it has also (dirty) oil outside. Cleaning an oily chain is practically impossible without first removing the oil by a solvent. Oiling an already oily and thus dirty chain carries the dirt inside the chain, destroying the chain in no time.
So, I repeat: only oil a chain that squeaks! Do not do any "preventive maintenance", or early oiling.
When shopping around for lubricant, what are the most important things to look for?
Thixotropy. Unfortunately, most marketers do not mention this property in the label of the product. However, if it is marketed as a motorcycle lubricant and is in a spray can, and if it is a wet lubricant as opposed to a dry lubricant, it very well could be thixotropic. You can test the thixotropy of a lubricant by agitating a spray can and spraying it to a transparent container. Then move the lubricant in the container by tilting. The lubricant should flow easily. Then leave the lubricant in the container to settle. Tilt the container after it has settled for a day. It should be very thick (but of course if you tilt it actively many times, it becomes gradually thin again due to thixotropy).
What do you use and why?
A thixotropic motorcycle chain lubricant, due to its thixotropy properties that mean it easily penetrates a chain after shaking the spray can, and when left to settle, it stays inside the chain. When starting to ride, it becomes thin again not resisting forwards motion of the bike, and when parking the bike, it becomes thick again, not dropping on the floor.
Avoid the so-called dry lubricants. They require constant reapplication even if riding in completely dry conditions. My wet chain lubricant lasts for hundreds of kilometers if not thousand.