I'd say that the largest problem with the OT from a technical plot point of view is the idea that the Millennium Falcon, without hyperdrive, manages to locate and fly to Lando's mining operation.
Think about this for a minute.
The only way that a ship without a hyperdrive would be able to get to another planet would be if that planet was in the same star system as it was. Accordingly, Bespin must be a planet in the Hoth system. However, this raises some questions. If the Hoth system was devoid of habitation, why didn't the rebels know about this operation, including Han and Leia? Furthermore, how convenient is it that this mining colony, located in the rebel's backyard, is home to Lando Calrissian, a friend to captain Solo and previous owner of the Falcon? The chances against that happening are literally astronomical.
Speaking of highly convenient occurrences, Yoda's death seems to require more explanation than is given in Jedi. As it is, he dies five minutes after Luke arrives on Dagobah, a stunning coincidence. It could be that it was his destiny to train Luke, and seeing as his training was complete, there was nothing to prolong his life. Or Luke's training took such a toll that he hung on long enough to tell Luke about his sister, then croaked. It could be that one of these hypotheses could be tastefully mentioned in the sequel trilogy as part of a character's greater understanding of the Force, but I'm not holding my breath.
On the subject of sisters, obviously Leia as Luke's sister was a misstep.
Another, less important issue concerns the Falcon's speed. In Star Wars, Han states that the Falcon is very fast, that she can make .5 past light speed. This seems to be a measure of her hyperdrive capabilities. Later, Lando also boasts that she's the fastest ship in the galaxy. However, in Jedi, Han says to Lando that the Falcon is the fastest ship in the fleet, and that would presumably help him in the battle over Endor. This implies that the Falcon is extremely fast at sublight travel as well, despite not being able to outrun a Star Destroyer at sublight speed. This isn't necessarily a problem, as the Falcon could be the fastest ship in the galaxy in hyperspace, while only being the fastest ship in the rebel fleet at sublight speed. Again, not as much a problem as it is a confusing technical distinction.
All of these problems except for the first aren't really problems in my book. The OT doesn't have all that many as far as I can tell.