to answer part of the original ? - I would guess the two biggest changes to the game are
#1 - fatigue and stamina used to be a major gameplanning weapon, such that the top coaches developed several techniques to wear out inferior teams, 1st fatigue was introduced to set rosters in addition to target minutes, but that still did not take this strategy away - then an engine change was made, which did, right now I don't think it is possible to wear another team out (a bad team / strategy can wear itself out still, gut a great team cannot wear out a good one simply by employing strategy)
#2 - as stated above, the game used to be a balance between recruiting, game planning, and practice planning, practice planning's affect has been limited due to the intro of potential, and recruiting's affect has been enhanced, making the player development part of the game largely a 3 day exercise (recruiting), rather than a 45 day exercise, recruiting and practice planning - I would guess top coaches were gaining 10 points a year (40 points a career) over the average coaches due to skillfull use of practice plans, right now, that number might be 5-10 points a career, it is still available, just not nearly as dramatic