The timeskip truly launched Roronoa Zoro, already an incredibly formidable swordsman, into even greater heights of power and fighting prowess. He trained with the world's greatest swordsman, Dracule Mihawk, for a period of two years, which drastically increased his improvements in skills and fighting ability.
Probably, the most striking feature of his post-timeskip appearance would have to be the massive scar running down over his left eye. Indeed, much speculation has been sparked among fans as to what hidden powers or significance it might hold because very few revelations concerning the nature and cause of this scar have been revealed. The real nature and cause of this scar remain yet to be exposed in the official storyline and add mystery to Zoro's development through the timeskip.
Now, this period of two years had seen much improvement in Zoro's strength and fighting skills. This could be indicated clearly from his return as he cut a ship into two pieces with his sword, much like he had done once before; similar to what Mihawk did. It had been through this show of strength that Zoro was in a position to show his great strides with Mihawk as his mentor, and that laid the foundation to become one of the major determining factors for the crew's victory in later fights.
Zoro's role within the Straw Hat crew had crystallized further during the post-timeskip period. He sealed his position as the main and first combatant of the crew, and the right-hand man to Luffy. With Luffy often absent, the burden of leadership usually falls on Zoro, who acts as the sword and protector of the crew. Hence, his changed role not only sees an increase in strength but also in responsibility and dedication towards the crew.
It is also after the timeskip that Zoro's fighting style had been refined to a great extent. Much quicker to grasp the rhythm of battle and more attentive as a fighter, he can now be more precise in analyzing opponents to find their weakness and make even more battle strategies. It had made him an even more powerful foe in combat situations with his already formidable physical strength and skillful swordsmanship.
This further extends toward improvements in Zoro's Haki, though in the past he has shown to have a little of it; his training with Mihawk made him master and polish such abilities. The ease with which he mastered Haki brought a new dimension to the way he fights and considerably boosted his offense and defense capabilities.
Zoro's development during and after the timeskip is a topic for debate among fans. Some think that post-timeskip Zoro is more interesting and complex as a character because increased power and responsibility added more dimension to his character. For others, elements of his pre-timeskip personality seem dialed back enough—most notably, his more frequent comedic moments—to change his character dynamic.
Though all of these arguments, Zoro's growth in the timeskip proved to be undeniable, which solidified him as one of the strongest and most respected members of the Straw Hat crew. Much of his strength and skills that he had gained at that time were instrumental in most post-timeskip battles. This often made him capable of taking down some of the strongest opponents they would have to face.
This timeskip seemed to focus more on Zoro's commitment to becoming the greatest swordsman in the world. Training from Mihawk, who was titled as such, gave Zoro a yardstick by which to measure his progress. Thus, he is further inflamed in his determination to see that dream realized. His increased focus finds expression in his more serious demeanor and his commitment to further training and self-improvement post timeskip.
It appears with the timeskip, Zoro's bonds with other crew members have grown stronger. He still has his stiff exterior, but through his actions, he demonstrates a great deal of loyalty and care for his fellow Straw Hats. One of the most important dynamics within the series—his rivalry with Sanji—continues to prevail even after the timeskip. Most of the time, this provided tension and comic relief in a serious crew.