I make a distinction between what I call to myself constructive and destructive grain. (Disclaimer: this is entirely subjective scientific nonsense.)
Constructive grain feels like it's superimposed on the image such that you can sort of look through it and see the all the sharpness, contrast and tonality of the untouched image. Terms like 'stochastic resonance' or 'constructive interference' come to mind (in human speak: it sings). Examples are Neopan 400 (Rodinal), TMY (Xtol) and, to a slightly lesser degree, Tri-x (Rodinal).
Destructive grain seems to keep you from seeing the image, it captures the attention in an ugly way. No matter how you look, the image just doesn't seem to come out. It doesn't shine through, somehow. Examples are Fomapan 400 and Kentmere 400 (Rodinal, Xtol). In 35mm I find Tri-x (Rodinal) also a bit destructive at times. Perhaps HP5+ (Xtol) also.
Mind you, the amount of grain is not important, it's the 'character' (shape, size, density distribution, whatever) that seems to matter to me. Neopan 400 can have a lot of grain in 35mm when developed in Rodinal, yet it still adds to the image. Use FP4+ in the same developer and it destroys everything.
Creating grain is not a huge trick. The hard part is getting out nice (i.e. constructive) grain.
Again, the above is utter nonsense in any objective, scientific sense, and I'm making it worse by throwing scientific terms into the mix. Pseudo science pur sang. But it is how I feel about it- call it a personal issue.