Hmmmm.....The first biggest change in action was going from a heavy cheapo clunker rod to a lighter rod, but that was pure delight. It was a St. Croix Pro Graphite, and I've bought much lighter rods since, and both medium and fast. None have given me much trouble in casting or fighting fish. My first bamboo rod was a 7' wt, and that rod did require more attention to set larger hooks. I lost several fish until I began using a strip strike on #10 streamers and Wooly Buggers. I have read nymphing articles in which the author expressed a preference for faster/stiffer rods, saying that slower rods dampen sensation enough to miss a take. I really admire Davie McPhail, and he uses a 9' 5wt Sage Z-Axis for non-indicator nymphing.
Of course almost everyone seems to develop a preference for rod action, so you might get responses like "I hate ____ action rods" and "I'll never try a ____ brand rod", but I'm always open to new stuff.