in bronze, ppl generally have no concept of personal safety, so you should play a kill lane and dominate your laning opponent and get super fed and carry (e.g. mid/top lane assassins)
in silver, ppl know not to walk into their deaths in lane (most of the time) so you should play a power farmer and outfarm your lane and win that way (e.g. mord, karthus, ad carry)
in gold, ppl start to get good at farming, so you should play a hero that can control the map and gank other lanes (e.g. TF, evelynn, junglers)
in plat you should just play your best role as often as possible and hope you are good enough at it to reach diamond (very few players are diamond level at multiple roles)
obviously there are a ton of exceptions... but these are the best strats at each level from what i've seen