Okposo's questionable hockey IQ, brutal decisions, and constant turnovers must have driven Tavares crazy. JT's best asset is his hockey instincts and intelligence, he can anticipate what most players are going to do at a high level. Notice how strong his chemistry was with Moulson because Moulson played a very honest game... go to the net and be ready. Even Parenteau was easy to read, he looked to get JT the puck with some open ice for him to work his magic. There wasn't much guessing so Tavares could just do his thing and while his wingers weren't super talented, they were easy to read.
Okposo loved handling the puck, he overhandles it to the point where he forgets that he has linemates sometimes. His decisions are so inconsistent, he free wheels it all the time. Tavares always had to hesitate because he had no freaking clue what Okposo was going to do most of the time. Considering that Tavares himself would try to do everything himself a lot of the time, it illustrated that he either didn't trust his linemates or he was frustrated with them because of the lack of chemistry.
Ladd is no superstar or anything, but he plays a more reliable North-South game that Tavares has a much easier time reading and anticipating how the play will develop. Ladd could help do some dirty work and win some puck battles (surprisingly something that Okposo was unable to do well in recent years) and crash the net and generally get out of Tavares' way. Okposo routinely seeked the same areas of ice that Tavares liked and got in his way all the time. Vanek was good with Tavares because he has good hockey IQ despite being a lazy bum.
Ladd and his style of game is much more complementary to Tavares and should mesh well. Okposo was a terrible fit for the reasons I stated and it was a big reason why Capuano put Okposo with Nielsen and oftentimes Bailey.