Not only would I disagree, but I would say the complete opposite. His rookie year, he was already an All-Star and helped us go from a 29 win team to a 48 win team. He continued to play great and had people doubting whether or not he was going to get it done (championship) at some point, but was never considered mediocre. Sure he improved a lot from dominant to unstoppable; but, hardly from mediocre to unstoppable. Now, fast forward two decades and you might find a Houston Rocket center that was considered mediocre that became the most dominant big man before his career was cut short. That one I can see.
Rodman was DPOY his 4th season (9.7 reb) and promptly averaged 12.7 reb his 5th season -- the same year he won his 2nd DPOY (1990). Pretty sure he was 'focusing on rebounds' that season seeing as he finished 2nd behind David Robinson. Not sure where you're coming from.
Tony Parker - fairly good role player to MVP candidate. Bruce Bowen Rajon Rondo - somewhat . . . Surprised no one has mentioned ... Sam Cassell.
Someone mentioned Nash, he's a good one. In that vein you have guys like Stockton and McHale who always put up strong rate #s but took a while to secure a starting job (Sam Cassell might also fall under that category).
I can't agree on Parker and Rondo. They both showed immense promise in their first and second years, and they gradually built up their careers into eventual stardom.