I personally think killing someone off was a great twist, and I don't hate that they killed of Nabu, they ensured we'd love him and feel for him when they did. (He and Brandon are my favorites as well!)
But I wish they handled Aisha/Layla's grief better, both hers and Riven's. They didn't even interact... and I'd think if you are both so close since you're all "one group" you'd have a moment to talk about your best friend to his girlfriend and vice versa, but there was nothing. She cried once and everyone bothered her in 5x01 and she saw a 35 second illusion of Nabu and that was it. It was poorly handled and I think that is why so many people end up feeling bad. A lot of season 4 problems truly only happen because 5 failed to deliver. The shop is gone, the earth fairies plot is properly developed and they ceased to even exist (except nebula for 1 single ep in season 6), Musa's solo contract (she had 1 solo and 1 with the winx) disappeared, jason queen disappeared, earth didn't change AT ALL despite now having fairies everywhere, supposedly...