I detest modes. Raids should be difficult, but not extremely hard.
One mode, for everyone.
This. A thousand times this.
Heavily stratified progression just kills a community. The hardest core content is only seen by a small minority of players. As people then lose interest, those same hardcore guilds have a hard time recruiting people because the commitment is so much higher than normal mode raiding.
One tier of difficulty for each raid. No raid that requires absolutely perfect tactics. A casual raiding guild should be able to clear all content within a reasonable amount of time (not immediately, but with practice).