>8-10 hour campaign
>Small-ish 30 minute levels
>Begins with a tutorial in Black Mesa where you play as kid Alyx sneaking around the dormitories, there are lots of cool little things to play with
>You're somewhere you shouldn't be when the resonance cascade hits, you have to find your parents and get evacuated
>Lots of sneaking and using contraptions to get past aliens and soldiers, very little if any combat
>You go to Xen briefly
>You witness your Eli's leg getting bitten off by a bullsquid and your mom's tragic death at the hands of marines
>G-Man extracts you before something similar happens to you
>You get to see the Combine invasion up close and personal
>A few small vignettes of the next few years under the Combine, Alyx is growing up. 
>G-Man keeps dropping tidbits, people start to learn about Gordon Freeman and somehow know he is returning
>Scavenging parts for Dog, going on resistance missions with him, starts learning how to shoot and defend herself
>G-Man returns and tells Alyx about Gordon
>Game ends with Alyx rescuing him from Metrocops