One of the things that jumps out at you with regards to "foldies" is that it's a design-heavy category, with apparent points for originality. Line up a bunch of road bikes or fully suspended mountain bikes and let your eyes go out of focus, and they all look the same. Not so with foldies. Every manufacturer has a different idea in mind as to what is important to the user of a folding bike. With this in mind, let's approach the issue from a different direction, to wit:
Why fold a bike in the first place?
Since folding a bike frame will either add weight or decrease strength or both, there should be a compelling reason (or reasons) for doing it, to make up for what is lost. Following, I write down the use-cases that I can think of for folding a bike. Maybe some readers of this post can comment this posting up with a few more. Anyway, let's begin:
- Fold up a bike to put it in your private vehicle, to avoid using a bike carrier rack: Perhaps you consider bike carrier racks a pain, and feel that they are flimsy and insecure. In this case, it's expected that ride quality will be important, because this use-case suggests you'll be getting your bike to the starting point of say a club ride, which could be of any length.
- Fold up a bike to carry it on a public conveyance that won't accept a full-size bike: In this case, it's expected that you will be attending your bicycle, and will presumably load it to and unload it from the conveyance by yourself. This use-case suggests public transit for commuting, so ride quality may be less important, as distances may well be shorter. A commuting scenario suggests that you'll be doing setup-fold and fold-setup sequences quite often, so compactness and ease of setup will be important here. And weight will be important, unless you're looking for some upper-body exercise.
- Fold up a bike so it can accompany you on a plane trip: Handing your bike over to someone else to handle requires a leap of faith, insurance to cover damage, or a highly protective case. Or very possibly all three. This use-case values ride quality over speed of break-down or setup; this isn't a commuting scenario, you'll likely be setting up and breaking down only once on the trip.
- Fold up a bike so that you can carry it to and store it in your office because you don't have a bike rack or other place to secure it: This scenario suggests an urban, multi-story setting. In this case, setup speed and weight of the bike would be important.
- Ease of setup/knockdown: How quickly can the user put the bike in a folded state from a rideable state or vice versa?
- Compactness: How small is the folded-state package?
- Ride quality: How well does the bike ride compared to a regular bike? What gear range is available?
- Weight: How heavy?
- Utility: Is the bike equipped (or equippable) with fenders or rack, or a special conveyance system (such as the Bike Friday carrying case / trailer)?
- Sizing: Can the bike accommodate tall or heavy/strong riders?