Since the dimensions are so huge, it is pretty much impossible that their society will realize they are actually sitting on a planet (instead of a huge, endless flat plane) until they have a comparable tech-level as today. Also see this question, about how a civilization would find out they are on a dyson sphere, the accepted answer about a 3AU dyson sphere. Your 1LY planet is still about 20000 times greater than the dyson sphere, but the similarities apply.
Compared to that, we on the other hand have already discovered that the earth is round in ancient Greek times -- we've known it for more than 2000 years. And we have known that we (earth) are not the center of the universe for more than 400 years. Meaning: your 1LY civilizations will have very... planet-centered religions, since they can't really imagine they are just one object amongst many in a huge cosmos (until they have reached at least 1950 technology and really great telescopes and really great scientists that can actually make sense of what the telescopes are telling them). Also, another hardship is that the physics (where the heck does a sun/moon come from on such a huge planet) must follow such complicated rules that we today can't even imagine them -- meaning their astronomy and exploration of the cosmos will be very, very delayed. Following from that will be, that a lot of technologies that have been developed for such cosmos explorations (first thing: space travel and sattelites) will get delayed a LOT.
Travel, on the other hand, should develop completely normally up to planes and other travel by air. However, space travel won't make a lot of sense until they are capable of bridging interstellar distances within a reasonable time.
The important step (where we currently are, trying to jump from orbit distances to interplanetary distances) will be pretty much useless to them, since they could just bridge the distance a lot easier in an airplane (at least you can land in the airplane, don't have to worry about exposure to cosmic radiation, no zero-G problems, etc.). If you compare the distance Earth-Mars (50-100 million kilometers when they are closest, let's say 1e8 km) to your 1LY planetary radius (ca. 1e13 km), you get that the radius is about 100,000 times greater than the measly Earth-Mars distance. That would be like traveling a couple hundred meters on earth. So, no, they really do not have any incentive of developing interplanetary-distance space flight for travel purposes -- they'd need to go for full interstellar right away.
You might see a need for interplanetary space travel though if you've got some strange objects orbiting your planet (like suns to make it light, for example?)
Planet-wide communication has the same problem as planet-wide travel: you need to be able to bridge interstellar distances within a reasonable amount of time. Because waiting for an answer for an entire (earth) year is not reasonable. So you need FTL technology -- and until you develop that, you've got isolated states/empires/... that can't really talk with each other, just like it was on earth before the invention of the telegraph. Travelers bringing news, hear-say of far-off countries, etc.
If you've got, for example, a desert on your 1LY planet that takes the same procentual space as the Sahara does on earth, it will be a completely insurmountable obstacle for hear-say communication spread with travel. Same with oceans if you just blow Earth geographics up to 1LY -- even a comparatively small ocean like the Caspian Sea will span distances from here to Pluto, not to mention the Atlantic or, worse, the Pacific. Oceans cannot be bridged in those dimensions. So, you would need two civilizations on both sides of the ocean who develop radio communication at approximately similar times, so that they can receive the radio waves from the other and realize: oh, there must be something beyond the Great Waters.
Ecology and Evolution
When considering such huge distances, you might get life evolving sentience in several places at once -- and in various different forms. Same for plant-life -- it might be a lot more diverse than we have on earth. Just travel a couple of AU (=distance earth-Sun), and you've got dinosaurs roaming the ground. Another couple AU later, you might have huge mammals like Mammoths and Sabertooth Tigers. Another couple AU later, insects might be the top of the food pyramid. And once you've gone another couple AUs, let's say, 60 AUs in total, you have still only covered a distance that would be around 12km on earth. Not even the stars will look different from there.
Planet-wide extinction events will be absolutely impossible, only local-sized.
Despite a technology level that might surpass us, they will be isolated societies comparable to countries in Midieval times. Yes, there will be trade amongst neighboring empires/states, and there will be ones who trade with countries even further away. But knowledge of all civilizations that exist around the globe (heck, even a rough map of the globe) will be pretty much impossible until technology for interstellar travel has been found.