I don't get the complaint that Picard was out of character, let alone enough to ruin the film (although this may be a little bit from watching it before "I, Borg" and finding that a less satisfying TBoBW sequel); encountering the Borg briefly twice (the latter time being Lore's pretty different version) in seven years doesn't seem enough for him to not want vengeance against them, especially when they seem increasingly likely to take over his ship and assimilate all of humanity.
That are my feelings as well. At the beginning of First Contact the Borg have completely assimilated Earth. The temporal wake let's Picard see this before he goes back in time. The stakes are super high for him. This is probably his worst fears come to life. And would anyone really just be "over" what happened to him. Sure he's dealt with it somewhat in the past, but that doesn't mean it's just gone from him now. The film shows how human he is.
If we really look at those other episodes after BOBW we have one injured Borg with emotions and in Decent we have a splintered group that are chaotic and emotional, doing none of the things they did in the past. First Contact is the first time we have the same Borg from BOBW.