I liked all the subtle Buffyverse references that dotted the series, for instance, a number of the clients of the Wolfram & Hart law firm from Angel were mentioned to be active corporations within the Alliance, (OOC: the makers of the old gun from Earth That Was were mentioned as W&H clients, which was great).
What was best though were the continuous background references to a mysterious plague on the planet Stoker, (Stoker!!!), during series three. A lot of them were really blink-and-you'll-miss-'em, like newscasters saying things like, "... symptoms include obsessive haemophagia ...". I still remember how obsessed the fan community got about potential cross-overs. But then, come series four, the references all stopped! Best red herring EVER!