There is no such thing as an ideal fly-rod because the performance characteristics depend upon personal preference. For instance the amount of flex of a fly-rod can determine whether the loop is wide or narrow, with a wide loop being preferred for delicate presentation and a narrow loop for distance or accuracy. The tighter the loop, the greater the amount of kinetic energy that is directed into the fly-line, and this allows the fly line to carry over a greater distance if required. A tip flex fly-rod will throw a tight loop whereas a full-flex action will create a wide loop. How a fly-rod flexes along its length is determined by the taper of its blanks. The wall thickness of the tip section dictates the strength of the fly-rod and hence the line weight that it can carry.
Resistance to flexing can influence the casting characteristics of a fly-rod, with a stiff action producing high rod-tip speeds but requiring more skill, whereas a soft action provides smoother casting and better damping, which is perceived as being more delicate. The resistance depends on the choice of woven carbon cloth that is used to construct the blanks and its property known as flexural modulus. A good quality, and hence more expensive, carbon cloth will mean that a thinner wall thickness can be used in the blank and hence the finished fishing rod will be relatively lighter. The cloth is impregnated with epoxy resin, which is wrapped in layers onto round, tapered formers and then heated so that it fuses together to make the rod blank
A very stiff action (broomstick) would be very difficult to cast with because smooth rather than shock loading is required at the beginning of the casting stroke, some tip action is required to turn the fly-line over and form a loop and damping is required after the stop, also to prevent shock loading.
There is the recovery rate from flexing to consider. The quicker a fly-rod recovers the quicker the amount of energy is transferred into the fly-line, which increases the conversion into kinetic energy and provides a force which focused in the direction of the cast (along the fly-line) giving better loft and a tighter loop. Needless to say this is of greater interest to distance casters, however a fast action fly-rod is also helpful in windy conditions, although the timing is more critical. On the other hand a slow action is better for casting lighter lines. Medium action is more forgiving and will still cast a line a reasonable distance with a good presentation, so this is probably the best choice for a beginner. Quicker recovery is influenced by designing fly-rod so that it flexes close to the butt as well as the tip section.
The hollow circular section of the fly-rod also gives good resistance to bending, compared with a flat section for instance, however if it becomes overloaded the circular shape will become oval and the wall will collapse.
Other factors come down to functionality. Long fly rods are used for casting heavy flies over long distances, whereas short ones would be more suitable for fishing with light tackle in small streams.
The shape of the handle can be used for applying pressure with the thumb, for long distance casting (half or full wells shape) or for guiding with the forefinger for precision casting (cigar shaped).