The first bad effect I ever noticed the first time through each movie were the long-range Tauntaun images in Empire Strikes back that looked obviously fake. They moved like claymation. The second was the Rancor which was of a different brightness level compared to Luke.
Otherwise, after having seeing Star Wars many, many times, I began noticing the weird boxes around the tie fighters (mostly when the Millenium Falcon is escaping the Death Star) and I began noticing some of the bigger stop-motion hiccups (like when Luke shut off his lightsaber on the Millenium Falcon or when Alderaan exploded).