(Note: I won't be watching the unaired pilot. For one thing, it isn't canon. For another thing: Riff Regan as Willow? No way.)
Short summary of episode: Buffy Summers, student-cum-Slayer, arrives at Sunnydale High, meets soon-to-be-sidekicks Willow and Xander, meets and rejects the offer of Watcher Giles, suffers setbacks when it's revealed by sphinxlike stranger Angel that Sunnydale lies on a stygian Hellmouth, goes to the sonorous Bronze where sidekicks are placed in suitable danger. With such stakes, she strikes against the blood-suckers until she meets strongman Luke who subdues her with a smash. "To Be Continued."
Analysis: With the first episode, there are certain things that will be lackluster: the special effects, some of the acting, even parts of the plot. But on the whole, this episode is better than most first episodes and this is why: it sets up the action quickly and, yes, while there is a lot of contrivance (Xander just happens to be in the library to overheard Buffy and Giles' conversation), the plot as a whole moves forward rapidly and with undue haste. The pace of the episode isn't slow, but it doesn't feel like it's rushing either.
The role of reversals is a big one, too: the opening scene is a reversal of a common trope in horror/slasher films. The blond girl walks down a dark corridor and is attacked by the killer/monster. But in this scene, the blond girl is the monster, which mirrors the later reveal that another blond girl fights monsters. Buffy's awakening is also mirrored in the awakening of the Master. And the characters are mirrors of each other: Cordelia is who Buffy once was, Willow is the opposite, the girl Buffy never was, but wants to hang out with now. Xander is the boy Buffy befriends (and only befriends, though he longs for more) and Jesse is his mirror (similar personality, but a different fate). The episode ends, though, predictably, with Buffy in danger (though this is perhaps a holdover from when this episode and the next were connected and shown as one pilot movie). A slight letdown from a less predictable pilot.
These themes (and more!) next week with "The Harvest."