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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For ideas. It needs a solid B. They introduced PoE which they crafted into a pretty good replacement for a raid. ToO was a good edition and you can't call ToO bad because all of Crucible is broken. As an idea without basis, it's great. All the little tid bits were nice too. New modifiers. Recrafting. Ascending. Etc. What could they have done better? Story, as is the problem with Destiny. ToO can get really frustrating. Even moreso than normal Crucible. A lot of potential they didn't touch