The big plot hole that always got me about "Datalore" was its claim that nobody knew who built Data, even though he was based on the positronic-brain principle that Noonien Soong was famous for advocating. That stood out to me as a self-contradiction the first time I saw the episode (or at least the second), and it's never gotten any clearer. And of course, it was retroactively made even dumber in "Brothers" when they cast Brent Spiner as Soong (instead of Keye Luke as originally intended). "Gee, this android we found has Noonien Soong's face and voice and is based on theories that only Soong believed in. We have no idea who possibly could have built him."
I also thought it was lazy and derivative to crib the concept of a positronic brain from Isaac Asimov. Aside from being imitative, it's a concept that even Asimov admitted was scientifically nonsensical. Although part of the reason I found it cheesy was that the really stupid Buck Rogers in the 25th Century episode "Shgoratchx!" had also cribbed the concept of positronic brains seven years earlier, and I'd felt it was an affront for such a dumb show to invoke the work of the sainted Asimov. (That season of Buck Rogers also had a character named Admiral Asimov, supposedly a descendant. I didn't know at the time that the new showrunner brought in for season 2 had worked with Asimov on an abortive film adaptation of I, Robot, so they actually knew each other and the homage was more legitimate than I'd assumed.) I didn't like seeing TNG sink to the same level as Buck Rogers (as I saw it).
There's also the fact that Data used one or two contractions in the first half of the episode but was suddenly asserted not to use contractions in the second half of the episode. That felt like very sloppy rewriting to me, like there were bits of different drafts in there and they hadn't managed to reconcile everything. Never mind that Data had used plenty of contractions in previous episodes.
