Any baseball fan likely realizes that the game is fundamentally different in 2013 than it was in, say, 1999 (the height of the "We Care About Steroids" era). In 1999, the league wOBA was .341; in 2013, it's .313. That's a titanic difference. Walks are at a 20-year low, and strikeouts this year are only lower than last year (and 19.7% of all PA). This seems to be understood by the vast, vast majority of Cubs fans.
On the other hand, there seems to be a disconnect at the team level. When people (myself included) look at a team that's offensively challenged, they have a tendency to overstate their putrescence. In reality, the Cubs are not much worse than the average team.
Darwin Barney is a defensive wizard, but he's an offensive black hole. Luckily for the Cubs, not only is the bar for 2B offense quite low, the Cubs have 2 internal options that might be able to cheaply fill in as soon as 2014. The first, Logan Watkins, has a .327 wOBA in the PCL, which translates (roughly) to a .297 mark in the majors, and Alcantara's .360 translates to .302. Either one offers a substantial dip in defensive value, but I'm not sure that the tradeoff of 60 points of wOBA isn't worth it.
The other places the Cubs could upgrade are relatively easy to fix. SS has been a problem this year, but Castro is still expected to rebound (and he's been improving) and I'd expect him to at least hit the average SS this year. If you believe in Junior Lake, you could hope he gets you .319 in wOBA: your other option is to buy wOBA in free agency (and I'll detail a few ways that could happen soon). Other than those spots, the Cubs are as good or better than the league average; the only person they have to replace here is Dioner Navarro, and even omitting him, Castillo's .312 wOBA is exactly average for the position.
If the Cubs wish to upgrade externally at 2B, RF, or CF (moving DeJesus to a corner – keep in mind you'd have to get a CF wOBA around .332 to offset the decreased RF production from Schierholtz), they've got a few options.
Robinson Cano is a free agent, and he's on a fringe-HOF path as it is. He's going to command somewhere in the realm of 8/$184, in my opinion. I wouldn't touch it, but he'd instantly take 2B wOBA up around 120 points. Kelly Johnson is probably a 2/$10 guy, which is totally fine, and he brings a roughly league-averag wOBA to the position this year and next. He's a solid replacement option that you don't feel bad about putting on the bench when/if Alcantara is ready to take it over. Chase Utley is probably being extended, and is probably priced out of our range for what he is.
The big get here is Jacoby Ellsbury, if the Red Sox don't re-sign him. He is probably good for around .335 in the wOBA department, with the fringe benefit of competent defense there. A quick and dirty guess at his contract would be 4/71, so right at 17.8 AAV. That's pricey, but that's free agency. You could try to buy low on Curtis Granderson, but I wouldn't be that comfortable projecting more than a .330-.340 wOBA from him with poor outfield defense. If you could get him for 2/18 or something, you could probably talk, but his name will get him more than that.
I've been carrying my torch for Shin-Soo Choo, and this is the reason why. He plays a sub-par RF, but he play it. He's on pace for a .388 wOBA season (and was projected for .372). I'd say the least optimistic projection for next year is .350, which is still 26 points better than the league average RF, AND you can keep the CF you already have (that's league average) in CF, where his bat plays. You could even move Choo to LF (which has lower offensive standards anyway, if not true historically), and keep Schierholtz there if you can find a platoon mate). Nelson Cruz is on option, too, but he's a horrific fielder, might be suspended/altered due to Biogenesis, is injury-prone, and is the kind of middling OBP – high SLG guy that we don't really need.
If you replace Barney with even a mediocre wOBA, you're sitting at a league-average offense in this depressed environment. The replacement is essentially free, with 2 valid options in-house for the league minimum. The Cubs have a ton of money to play with, and there are options in free agency to cover their few holes offensively. Add in a return to form somewhat for Castro, a step forward from Rizzo, and some natural regression to the Cubs RISP BA, and it's not hard to envision a Cubs team that is at worst an average offensive club.
Fun with MLE calculators: Javier Baez' projected line in the majors off of his AA stint is .187/.237/.546, 35.1 K%, 6.5 BB%.