If I can speak on behalf of liberals... I think they don't view America as a bad place... instead as a good place with faults, that could be even better by learning from others that have had successes where we haven't. Unfortunately that gets painted as "America sux" often times. Though, admittedly, there probably are a lot of extremist liberals who do believe that, I don't know.
Example... I think we should go to a single payer system for healthcare, and I think our healthcare is disappointing relative to certain other countries for the majority of people... BUT relative to another set of countries, we have it pretty well here, AND we are at the forefront of many advancements in bio-tech. Being aware of our shortcomings, being critical of them, and wanting to improve upon them, doesn't necessarily mean I think America is terrible in that field, just that we could be even better.
As far as greatness... I'm of the general opinion that every generation is better* than the previous generation in that we continue to build on successes of the past. A lot of amazing things happened in the past, and a lot of shameful things as well. The idea is to remove the shameful things, or at least learn from them, and then also build on the amazing things. We don't necessarily put people on the moon anymore, but we just got some sweet pictures of Pluto and are discovering new things about space every day. Some look at that and say "we aren't as great" others say "well, what more do we gain from putting people on the moon? Let's see what else is out there, and what we can learn from it"
* - its pretty difficult for me to say one generation is hard working, and another isn't, just because we now have protections for those who can't provide for themselves... in the past those people suffered greatly, and were not able to turn it around very often. Now with a social safety net, there is a greater possibility. At least theoretically, since it's a topic of great debate.