Horrible mistake to miss Dubrovnik?

You have made a valid decision, since Split and Duvrovnik may have the best public transportation and touring options. Don't expect to find a sleepy, undiscovered village when you get to Split, however. You might want to make sure that Mostar is just as accessible from Split as from Dubrovnik, from which we went to Mostar (by organized tour.) Regarding Dubrovnik, I'd remind you of the Yogi Berra-ism, "Nobody goes there anymore, it's too crowded."

If you're including Plitvice and Zagreb, then absolutely, it makes sense to use Split as your base. You can't see the whole country on one trip, so no shame in missing Dubrovnik at all. Do what makes most sense logistically with the time and constraints you have.

What are the dates for your trip? How many nights will you have in Croatia? It is never a mistake to develop an itinerary that makes sense geographically and allows you enough time to see the places you choose to visit.

I think the following basic route would work, and I don't see anywhere you would need a private driver or a tour. However, there might be an attractive one-day tour from Split that would get you to multiple interesting places hard to combine via public transportation.

Fly into Split.
See Split, with possible day-trips to Sibenik (train? or bus), Zadar (bus), or Brac (ferry).
Bus to Plitvice in the afternoon. Possibly see part of the park late that day
Plitvice. Finish with park. (You'll probably want a total of 4 - 6 hours there.) Bus to Zagreb.
Zagreb.

I haven't been to Sibenik (recommended by another poster here) or Brac. I quite liked Zadar. The medieval center is on a peninsula; it's a bit of a walk from the bus station. Zadar is popular with European tourists, but I still enjoyed several hours walking around after arriving late and spending the night there. Zadar has two worthwhile contemporary monuments on the waterfront, one of which (Monument to the Sun) needs to be seen after dark, which might not work out on a day trip.

Don't try to travel to Plitvice, arrive in the morning or right after lunch, and see the park then. It will be nightmarishly crowded with day-trippers. It's a great, great destination; see it at its best by avoiding the mid-day mob-scene. This is not the place you want to visit on a day-trip!

I like Zagreb a lot. It has a large, bi-level historic district, an active café culture, a bunch of good, traditional museums, and two excellent unusual ones: The Museum of Naïve Art and the Museum of Broken Relationships.

Croatian buses are comfortable. You can Google for schedules, but where buses are concerned, I like to double-check once my feet are on the ground in the departure city. If I arrive by bus, I check the outbound schedule before leaving the bus station. Pay attention to days of the week, and be sure you know of any holidays falling within the period of your visit.

Don't be surprised if you're asked to pay the equivalent of about $1 US to place each suitcase in the hold beneath the bus. I think Croatia is one of the countries where I encountered that.

Personally? Yes, I think it would be a "mistake" to skip Dubrovnik while you are so close. It's one of the most unique places I've been. True, it's pretty small and often crowded with tourists during the day. The tourists can be obnoxious. But in terms of unforgettable places, I'd certainly rank it above Split/Trogir and Zagreb. Mostar is different from any of the other places, but if I had to choose Mostar vs. Dubrovnik, I'd choose Dubrovnik despite the tourists. (Mostar isn't much more travel time than Dubrovnik.) Plitvice is something other than a city or town of course.

Then again, I probably wouldn't day trip to Dubrovnik from Split, either. Dubrovnik is best seen at dusk and night to get the full experience. I'd consider staying a night in Dubrovnik then flying to Zagreb from there - but not sure how you'd fit Plitvice in then.

I've been to Croatia 7 times and never been to Dubrovnik. I don't feel bad about it at all. I'm not really a fan of Zagreb, though. Is there a reason why you won't drive? It makes everything so much more efficient and the roads are excellent.

I'd echo lots of the above, but...honestly, I'd say that choosing to do Croatia entirely by public transportation is the bigger mistake (assuming there's not some reason that you absolutely can not drive a rental car). Sure, you can do almost anywhere by public transportation if you work hard enough and make enough compromises, but IME Croatia is not a place that works very well traveling entirely in someone else's vehicle - at least not efficiently. There are lots of amazing places you can get to (and away from) quickly and easily if you have a car. Yes, there are some places (just a few, actually) where having a car is an inconvenience, but I think for most Rick Steves type travelers, Croatia is best done (at least partially) with a rental car. It's not like going to other popular places in Europe where travel 100% by public transit is easy and smart.

To your specific question: Dubrovnik is amazing, and I really enjoyed my time there even though I detest crowds (and yes, you will find horrible crowds there, although there are the usual ways to mitigate them - acraven's comments above about Plitvice are spot-on, apply them to any place that is being loved-to-death by tourist crowds; he also asked the critical question about when you were planing to go..).

So yes, IMHO it seems like a mistake to me to skip Dubrovnik when you're right there.

You have not said how much time you have in total, or if you have searched open jaw flights into Dubrovnik and out of Zagreb.
Without knowing that, I can't answer if it is an option, but I certainly do not think it is a tragedy to skip Dubrovnik.

Driving in Croatia is easy, very easy. There are big toll roads. These are like autobahns anywhere. There are smaller roads. These too are easy to manage.

The only places I fear to drive in Croatia are in the center of old cities. Not fun. Crowded, tight spaces, nutty other drivers. Split old town is very difficult for the non-native driver. So, if you want to go, I would drive to Dubrovnik.

However, Dubrovnik is part of the "coastal city" group in Croatia. Dubrovnik is the best. Trogir, Sibenik, Zadar are all along the same plan. Not as big, not as complete, but the same basic idea. If you do Zadar, Trogir, and Sibenik, Dubrovnik will not be missed as much.

This is my question, too. Most of my trip is built around meeting up with friends and family, includes a road trip ending up in Trogir and Split. I have been thinking that I would just entertain myself in Split for 3 days/4 nights while companion works but everyone keeps telling me I should go on to Dubrovnik for one night since I will be so close. I tried getting flight out of Dubrovnik but that didn't work with family/work obligations so I would have to go back to Split.

lala - You don't say how long your trip is (or where else you'll be going), but 3-4 days in Split seems excessive to me. Actually, with 4 days, I'd spend one in Split, head to Dubrovnik, spend a day or two there, and I'd take a day-trip to Kotor, too (unless you have already been or will be going back to these places sometime soon).

I ended up going to Dubrovnik because I really had that in my mind as soon as we started talking about Croatia. I did love it, but it was exhausting to go there just for an overnight. I felt that I had plenty of time to see what I wanted to there, loved walking the walls in the late afternoon, wandering around the narrow streets in the evening...but the next morning, I got sick of the crowds immediately and took an earlier bus out than I had planned. If I had been able to stay longer, I would have enjoyed going to the beach or doing a kayaking trip, but I probably wouldn't have needed to spend any more time on sightseeing in Dubrovnik itself.
I agree with you that it is great to settle in for a few days and explore an area, so I would not try to be rushing from one place to another. Croatia seemed like a great place to relax at the beach and explore more at each place. We didn't have enough time to do that (since the schedule was really dictated by a family member's business trip.)
I enjoyed Split and Trogir as well. If I could have planned the whole thing myself, I would have gone for longer, had more nights in each place, more beach time, more sitting around at Riva cafes, more boat trips, etc.

But lala, the question is: are you glad you visited Dubrovnik even as rushed as it was? (There are a number of places I felt I just HAD to go once I got them into my mind, and some were great some not so much. Dubrovnik would be closer to the "great" side.) I agree, it can be really unpleasant with all the tourist mobs. But I LOVE it at night especially when it is all lit up and so much less crowded. (Did you make it to the Buza Bar to see sunset? How was the weather?) That's why I usually recommend to people that they plan day trips if they are staying in Dubrovnik - don't plan to hang around and "see the town." Or go to the beach during the days.

Yes, Andrew, I am glad I visited Dubrovnik. It really is an amazing place. But going back to the original question, would it be a horrible mistake to miss it? No, I don't think it would be a huge mistake, just a decision that needs to be made balancing the various other places to go, the time available, etc.