Saying Isiah's defense is a negative is a pretty big understatement, imo. Is he a good player? Yes, but he also happens to be a volume shooter who often plays out of control and is pretty selfish, to be honest, especially for a point guard.
Volume shooter - how?
He only averaged 13 FGA per game this season. Take a look at all the other players in the league who are averaging around 19 PPG and I think you'll find very few (if any) who have done so on < 14 FGA per game.
Thomas is an extremely efficient scorer in terms of the number of points he scores relative to the number of FGA it takes for him to get those points. His scoring efficiency (PTS / FGA) of 0.4 is not only among the highest in the NBA right now, it's among some of the highest numbers in NBA history. That's
specially impressive given he's a PG, because that's usually the least efficient offensive position on the court.
With the exception of Chris Paul, John Stockton and Magic Johnson (maybe Steve Nash?) I don't know of many other Point Guards who have had that type of offensive efficiency.
As for being selfish and playing out of control, I don't see that either.
He has an assist-to-turnover ratio of 2.08 which is pretty respectable for an NBA PG, especially one with a shoot-first mentality. In fact, he ranks 20th among all NBA PG's in that category. His number is in the ballpark of guys like Derek Rose, Deron Williams, Kyrie Irving, Tony Parker and Jeff Teague...and his scoring efficiency is better than any of those guys.
When I see him play, I often see him pass up average or good shots for a teammate who has a better shot. I've also seen teams throw almost endless traps and presses at him, and I'm not sure if I've seem him lose the ball once as a result.
He's a better offensive player than many give him credit for - people get overly caught up on his size and his perceived defensive issues (which are legit weaknesses) to the point where they don't really recognize how good he is on the other end of the court.
26.4 / 7.5 / 2.9
Those are Thomas' Per-36 stats. He doesn't get those stats because he doesn't play the minutes, and he doesn't play the minutes because of his size and what that brings. But don't make the mistake of underestimating his skills - if he was 6'3" he'd be an all out superstar, easily Kyrie Irving good.