I started out kind of slowly in this one, but once I got to the middle, I put in almost every Across answer immediately, and, well, then it was over. Now let's see if I can figure out the theme... Aha! It's a progression from "crawler" to "flyer!" Lovely. We start with my old favorite search engine WEBCRAWLER (17A: Bot that systematically browses the internet) ("browses" might be pushing it), then we get to ALICEWALKER (28A: "The Color Purple" novelist), then BLADERUNNER (48A: 1982 Harrison Ford sci-fi film), and finally RADIOFLYER (64A: Classic red wagon). I like it. I like it a lot. Four perfectly normal entries combined in a sensible, interesting, and fun way.
On the non-theme plus side, however, we've got EMERALDS, EMBARKED, SERGEANT (not abbreviated!), ETCETERA (ditto!), KEMOSABE, VERBOSE (46D: Wordy), and URBANE (51D: Polite and refined). That's a lot of great, middle-length stuff right there.
Good start to the week!