"I want a fast action rod that's not too stiff..."
WOW! There's a fella who knows exactly what he wants in a fly rod. For most, however, it's just not that easy when you are trying to spend your hard earned money wisely. How well a rod will cast and fight fish in a particular environment is key, so a bit about rod actions and casting characteristics is worthwhile.
The action of a rod is determined by where it bends with a given amount of weight. Fast action rods bend nearer the tip area and medium action rods bend down nearer the middle of the rod. Slower action rods bend all the way down into the butt section and look parabolic when bent with that same given amount of weight. All you really need to know is that faster action rods are stiff in the butt and flexible in the tip, because you will use a great range of weight when you cast varying distances or when using larger/heavier flies.
Per AFFTA, our industry organization's standard for the first 30' of a 6 weight flyline is 160 grains, or about 3/8 ounce. But if you are casting with half that much or 15' of flyline (80 grains) - the AFFTA 30' weight standard say 80 grains is a 2 weight! Or, let's say you are a pretty good distance caster and you need your 6 weight to carry 70' of line to shoot a presentation cast to 100'. We know that the first 30' of our 6 weight line weighs 160 grains, but running line - at approximately 3 grains per foot - weighs another 120 grains (40' x 3 grain/foot) for a total weight of 280 grains of flyline - AFFTA 30' standards say 280 grains is a 10 weight! As Lefty says "all flyrods are a compromise". The need from tip to butt in Medium Fast, Fast and Very Fast Actions....pick what suits your casting style, or if you are just learning, pick an action based on the distance you plan to cast.
Personally, we think the best advice is that it's just fishing....so pick a rod, get in the sport and go have some fun. We'll be here to help if you have any questions.