For some peculiar reason, we equate "All-Star" with "winning" in the NBA, often forgetting that basketball is a team game and that there are very few players in the league who are single-handedly capable of leading a team to the postseason—or even a winning season.
There are a few, but very few. If we made the All-Star game consist solely of those players, we would't be able to field two starting fives.
Some players that are "All-Stars" aren't as good as players that aren't "All-Stars." They earn the moniker for no better reason other than they happen to have better teammates.
There is a short list of players in the NBA who, if they were playing for a better team, would be no-brainers. Yet because they are toiling with losing teams, they are overlooked.
In each of these cases, the player in question has played for a bad team and not made the All-Star game. If you were to put them on a winning team, they'd probably be in. If they were on a team contending for a title, they'd be a shoo-in.
Note: These are not players who are underrated; it's players who haven't made All-Star games because they're on bad teams. Before suggesting omitted players ask two questions. 1) Were they on wining teams, and 2) Have they made an All-Star game. The "Ty Lawsons" and "Kevin Loves" of the league are not on this list because they didn't meet the criteria.