I would have to rate HoW a B- / C+. ToO is excellent, PoE is less than good, especially the weapons, the rewards / effort trade-off of PoE Lvl 35 and lack of matchmaking for at least levels 32 and 34.
Edit: A historical note on grading:
Some schools do hand out E letter grades instead of an F, but they are in the minority. A majority of schools give grades of A, B, C, D, or F.
Rather than a failure on the part of academic institutions to know the alphabet, the simple answer is that “F” stands for “fail.” The other four grades are more or less considered “passing” (though in some districts a D is also a failing grade), which is why they go in alphabetical order. The F is considered separate as it denotes a failing grade, and does not need to go in alphabetical order. It just so happens that “fail” starts with a letter that skips one letter alphabetically on the scale.
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It goes beyond just PoE and ToO. It's about fixing the mess that was TDB. They finally are starting to make changes that make sense (uses for npcs, weapon reforging, new weapon class, new weapon talents, minimizing all these stupid currencies, crafting bosses that are more than just bullet sponges that actually have somewhat creative mechanics, showing pvp players some love, etc... For that reason I gave it a B. Just the content itself was a C/D, but the general way that the game is headed is very promising and they went a long ways towards making Destiny a better game in the future.