The Indians and Cubs have many similarities in terms of their team-building. The Indians drafted and developed a number of their top players (including Francisco Lindor and Jason Kipnis), along with a number they acquired as minor leaguers via trade such as Corey Kluber and Carlos Santana), and some low-cost additions through free agency (like Mike Napoli and Rajai Davis). The Cubs also drafted and developed a number of their stars (Kris Bryant and Javier Baez) and made some seemingly minor trades (acquiring Jake Arrieta and Kyle Hendricks, among others), but they were able to augment their roster with massive free-agent signings (Jason Heyward and Jon Lester come to mind), the likes of which the Indians have never experienced.
Each team put together a strong starting rotation, though the Cubs have been more fortunate with the health of their group in recent months. Each team traded a very talented group of prospects to acquire their top relief pitcher from the Yankees, first with Chicago acquiring Aroldis Chapman, followed shortly by Cleveland adding Andrew Miller. Each team has constructed a roster they'll mostly be able to keep intact for at least another couple seasons after this one.
Another thing the Indians and Cubs have in common, is great defense.
Roberto Perez is one of the best defensive catchers in baseball, both in terms of his ability to limit the opponents's running game, and in terms of his pitch-framing. The Cubs have three different catchers on their World Series roster, each of whom has gotten significant playing time already this postseason. Miguel Montero isn't great at limiting the running game, but is an outstanding pitch-framer, maybe the best in the league this season. Willson Contreras is the opposite, only a decent pitch-framer, but very good at limiting the running game. David Ross does both things very well.
Indians fans know how good Lindor's glove is, but Cubs shortstop Addison Russell's defense is almost as good. Meanwhile, Cubs first baseman Anthony Rizzo, second baseman Baez , and third baseman Bryant are each better defensively at their position than Mike Napoli/Carlos Santana, Jason Kipnis, and Jose Ramirez are. On the whole, while it's close, Chicago has the slightly better infield defense, with each group among the top three or four in baseball.
The outfield though, is a different story. The Indians are very middle-of-the-road defensively in the outfield, with the specifics depending on which three players they have out there on a given day. The Cubs, on the other hand, led by Jason Heyward's fantastic range in right field, rate as one of the two best defensive outfields in baseball.
Putting everything together, the Indians have been one of the better defensive teams in baseball this season, but the Cubs have been the very best. Neither team is prone to making mistakes, but no team can match Chicago's ability to turn ground balls, line drives, and long flies into outs. Combine that with their excellent starting rotation, and the Cubs are a very tough team to score against as evidenced by their collective ERA of 3.15 having been (by a wide margin) the best by any team this season, and their collective ERA- (ERA adjusted for era, park factors, etc.) of 76 having been the best by any team this century.
Draw walks and hit home runs, the Indians will need to do everything in their power to take Chicago's defense out of the equation, and Tribe pitching will need maintain their postseason excellence for another four to seven games.