So, how does a game end up with a '1' in its title anyway? Zork began its life as a much larger game on Mainframe computers. After it was decided to exploit its potential commercially, regular home computers became the limiting target. So Zork was split into three parts - this one being the first.
At the time, Zork's parser was revolutionary. The common standard of Adventure games was VERB NOUN parsers in those days. Zork came much closer to understanding natural language, even the combination certain commands into full sentences was possible. This, however, doesn't win the game anymore points these days. How does it hold up stripped of that advantage over the competition?
ZORK I shows its roots in the cave exploration/treasure hunt genre. Without further explanation, the nameless protagonist enters a house and finds a system of underground caves below. He has to explore it and retrieve as many treasures as possible. This includes various subquests which can be played more or less independently from each other. This is a useful approach, because a(nother) thief is roaming the caves as well, just waiting to snatch things away from you. Sooner or later, you'll have to get rid off him, too. In the meantime, storing your treasures securely in the house before going down again seems sensible.
The game is written with a good sense of irony. In this context, the game is quite self-aware, breaking the fourth wall regularly. The background of the IT university labs is very much visible in the allusions and jokes - the more similar one's own background, the better.
Puzzles are mostly self-contained, but still original and anything but trivial. Questionable in this respect: You'll encounter random elements regularly. Also, there is an overall time limit, governed by your brass lantern simply failing after a (generous) amount of overall time.
These are minor annoyances at best, though, because they are implemented in a way which doesn't frustrate the player too much, either by occuring too often or having too strong an impact. Zork is still an entertaining treasure hunt - non-pretentious fun.
Interesting historical tidbit: Infocom actually tried to translate the underlying engine and Zork itself into German at one point. The project never escaped early beta status, though, most likely due to German grammar being harder to parse in a canonical way than the English one. The unfinished beta version appeared on the Internet many years later. It contains some hilariously clunky prose and switches back to English right in the middle.