depends on how you define "effects"
Is it how the game is played- then it is easily rule changes.
If It is popularity then TV hands down.
If it is quality of the game then it is a combination FA and Cap. This prevents building and just as importantly maintaining a real dynasty. The Pats teams that have won SBs would have been worked by qualitatively better teams like the Boys of the 90's. Nuts the Bill's of the 90's would have eaten current Patriot teams alive. You just cannot get the kind of talent on a team let alone keep it like we did in 1990's. If there had been no cap that 1992 team would have been around for at least another 5 years. As it was it began to degrade very quickly due to FA losses. Which were due to the Cap since we know Jerry would have spent what was necessary to keep that team together.