For me it's when all vocals in the song get almost the same amount of lyrics if there are more than 1 vocalist. So what I mean is not when a DJ collabs with a singer, but when a band with a vocalist would collab with another vocalist or if a 3 people group collabs with a band that has 2 vocalists and every has almost the same amount of lines.
For bad features I do dislike it when you barly even hear the featured vocalists and don't even notice their existence in the song. So for example a 4:30 minutes song could have a total of 3:50 vocals. 2 people sing in the song, 1 has 3:00 of vocals and the feature only 50 seconds. Depending on context of the lyrics it may makes sense, but if its a song where you know both could have almost the same amount of lines, than it sounds very bad to me, especially if both vocalists might also have almost the same vocal tone or a very good harmony with eachother.
So for a good collab it could look like: the band has 2 vocalists, the featured artist is 1 person. The song is 4:00 and has 3:10 lyrics. The 1st vocalist gets 1:10 lines, the 2nd vocalist 1:00 and the 3rd 1:00 of the lines. That looks like a pretty good harmony to me and if they even have similiar voice, the better. But it would suck as a bad collab if for example from the 3:10 minutes the 1st vocalist had 1:30 the 2nd one also 1:30 but the feature would only get 0:10 of the lyrics. That would truely suck for me.
If the feature is a musician with a distinguished sound (not a DJ) than it would also suck if for example the guitarist who got featured is barly to be heard like example you wait for an epic guitar solo but you almost her no guitar in the entire song and also no guitar solo and you think: "is there even a guitar in the song?"
For DJs I think it mathers more if the song is enjoyable. If they have a personal style than it looks even better if they come up with a very great idea to use the vocals in the song that they get featured on or that they want to have as feature. I mean imagine the David Guetta-style but he would make a song for a metalband, that would never work out. But if he for example would decide to feature someone like Zara Larsson, than it most likly works pretty well at the end. However the song could totally be change and be unenjoyable if Zara would say no and he had to try to come up with another sing that suits the song.
