Well he was a good leader for the Southland, particularly in taking advice of his ministers, was a decisive leader, made sure the Southland was stable throughout his reign and was able to make the best use of his advisers such as Zhou Yu or Lu Xun.
Its not really the job of Sun Quan to personally lead troops into battle, thats what generals are for. The real value of a leader in Sun Quan, Cao Cao or Liu Bei's position is their ability to use people. Thats what makes them able leaders. An prime historical example is the Chu-Han contention which led to the founding of the Han Dynasty. Liu Bang knew how to make use of his advisors while Xiang Yu mainly relied on his own might and power and in the end Liu Bang himself said it was Xiang Yu's inability to use his men properly which led to his downfall.
On expanding the kingdom, thats a whole debate in itself whether Sun Quan really wanted to expand Wu quickly. Remember Sun Ce's advice to Quan on his deathbed was that he should hold the Southland, because thats what he was good at and not go quickly to expand Wu's domain. Another factor to take into account is when Sun Quan was finally ready to take over China, it was a highly difficult task because Liu Bei and Cao Cao had both established themselves. Compare to when Cao Cao took the northern heartland he was fighting amongst multiple warlords whom all wanted to take everyone else out. Its much more difficult to win against one well-established foe than win against a pack of warlords who aren't looking out for each other.
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