Not hatred. Dislike. Dislike is fine and healthy. If I ever encounter someone who like everything, I'm going to think something is wrong with that person. There's a lot in this world that people should dislike.
Expressing dislike for an edition and saying something like, "Well, I don't like 4e, because it doesn't feel like D&D to me." is not bad, nor should it cause the person who loves 4e to feel hurt or attacked.
Where things go wrong is when the person who dislikes 4e takes it a step or two further and instead says something like, "You like 4e!? 4e is a piece of crap and I can't believe you would like it!" There, instead of just expressing your equally valid opinion on the quality of 4e, you are making it an attack on the person.
Upthread
@Umbran gave the analogy of the 5 year old and that analogy doesn't work. In his analogy the 5 year old is invested as the creator of the piece in the same way that the designers of 4e are. I would expect the game designers to be more upset over someone disliking 4e to the extent that it didn't feel like D&D to them, than player X. To give a real life example. The other day my 6 year old was watching some kids show and he asked me if I liked it. I told him no, because it really wasn't my thing. He shrugged and said, "Well, I do." To which I replied, "It's okay if you like something and I don't." On the other hand if he had drawn a picture and came running proudly up to show it to me and I didn't like it, his face would fall. The level of investment is different.
And to say one more thing, if I dislike something and am not allowed to express my feelings, that's harmful to me. Being forced to keep feelings bottled up, because someone else has expressed that they like something is also bad. People should be able to express both like and dislike, and express the reasons behind their feelings, without it harming the other side. They just have to be expressed in a manner that is not also attacking the other side.