Of the young actors, I thought they all had good moments, but Daniel and Emma also had flat moments. Rupert Grinch I think is shaping up to be the best of them, and his physical presence and energy was marked. In fact in this film I felt that the Weasleys in general had a vitality which was missing from Harry and Hermione, and that this was a deliberate theme. Harry is described by Dumbledore as 'unfailingly kind' and the downside of this - inability to dominate, difficulty in making ruthless decisions - runs right through the film. It perhaps makes the film dull for people who would prefer a more focussed and dynamic hero, but that's the story.
Another character finding it difficult to follow through on ruthlessness is Draco. I find this actor (forget his name) a competent one-note, but in this film his repertoire extends to match his increased ability, and he is incorporated into bleak monochrome scenery as a bleak monochrome item, which worked well.
Of the adults, Slughorn is played by Jim Broadbent, at a range of ages, and I thought he did a very good job. What else would we expect? I thought his dynamic - continually weak and tempted, barely holding himself back from the darkside through personal vanity - came across with greater clarity than in the book. As well as the dichotomy between kindness and ruthlessness, a second opposition is between intuition and rules as guides to action. While both Harry and Hermione are kind, Harry and Snape are intuitive, while Hermione and Slughorn use rules. It was only in this film that it was obvious to me that the opposition between the text book printed words, and the scribbled marginalia, is a way of symbolising this dichotomy. The different approaches of Snape and Slughorn to both potions and the dark side.
I'm afraid I thought Michael Gambon didn't quite get it right at times. He's obviously very good, and he conveys authority and charisma well. In some scenes where it was crucial he is clearly weak and weakening I don't think this was always portrayed compellingly. But perhaps this is a flaw in the narrative, where the same character must be weak and dominating at once.