I'm a little surprised by how many people think this trade makes the Nets a lot worse. I don't think they'll get better necessarily, but Lopez is a bad defender, bad rebounder, and kind of a black hole. I expect they'll wind up with roughly the same record with Russell (as he continues to develop and figures out his new role in Brooklyn) as they would've with Lopez.
I think it's a great move for Brooklyn long-term, but I do think they're worse next year.
They're both offense-only players, but Brook is also HUGE. He averaged 1.7 blocks a game just due to his size and length. Russell, on the other hand, brings virtually nothing on the defensive side of the ball, along with less overall offense in this stage of his career.
I don't think it will be some major drop-off, but I do think they're a good deal worse next year without Lopez's veteran leadership, offense, and presence in the middle.
I do really love Russell's fit in that offensive system, though. Really excellent long-term pickup for them.