The teddy bear that Bruce Willis brings to his son is the same bear Alec Baldwin brings home to his daughter at the end of "The Hunt for Red October," which is another John McTiernan film.
At the end, Karl emerges from the building and tries to kill McClane, only to be gunned down by Powell. The triumphant music which swells as Powell shoots him is from the "Aliens" score--it plays at the end of that movie when Bishop reappears in the dropship to save Ripley and Newt, after they think he's abandoned them.
Die Hard and Die Hard 2 are based on books by separate authors, "Nothing Lasts Forever" by Roderick Thorp and "58 Minutes" by Walter Wager, which share none of the same characters. In both books, it is the hero's daughter that is in peril rather than his wife, and the hero is also more trusted by the authorities than McClane. (In "Nothing Lasts Forever," he tosses his wallet out the window so the police on the ground know he's a cop.).
Regarding a previous Trivia entry: in the German version, ALL German terrorists received Anglicized names, and their background was altered as well. This was due to terrorist activities in Germany - the kidnapping of an airplane in Landshut (1977) and the activities of the Rote-Armee-Fraktion ('Red Army Fraction') in the Seventies -; the German producers evidently felt uncomfortable about the idea of having German terrorists appear in foreign movies also.
The terrorists show up at Nakatomi Plaza in a truck marked "Pacific Courier" (except Karl and Theo, who arrive by car). This fictional company name also appears on the side of the plane the bus crashes into at the end of "Speed." Jan de Bont, who directed "Speed," was DP on "Die Hard."