I was thinking about this question after a visit to the Pacific Pinball Museum (which was great). Why were some games fun to play and others weren't? I came to a few conclusions about what principles are important. El Dorado was the biggest lesson (and biggest surprise) about how much more fun a good game can be than a bad one.
1) Like you said: clear objectives! I would say even "self-evident" objectives. Nobody wants to read a bunch of tiny text on the playfield or on the instruction card -- you want to be able to intuit what the game wants you to do and react in real time (see #6 below).
2) Use the whole playfield. So many games have three or more pop bumpers at the top of the playfield, often in an area in which there is no meaningful way to earn points. They're just filling space. There is no reason to shoot into them, and no way to guess the outcome if you do. Even though pop bumpers can be super fun if used well, they can also create a meaningless division of the playfield, where one zone is to be avoided. That's no fun! For contrast, take El Dorado. That row of drops along the top, and the extra flippers up there, ensure that the fun extends all the way up to the top of the arch.
3) Some left-side/right-side asymmetry on the playfield. El Dorado is a good example again -- you have to adjust your strategy on the fly based on which side you have to work with, how much is left, where the ball will likely end up, etc. The least interesting games, in my opinion, are often those with completely symmetrical playfields (Jungle King I'm looking at you). There are of course exceptions (2001 comes to mind -- super fun).
4) No sense of unfairness. Unfairness is the anti-fun. The player should always feel like she had a chance to accomplish the unlikely, or at least delay the inevitable. Even if a game is a drain monster, fairness (or the illusion of fairness) will turn that difficulty into fun.
5) Related to fairness: scoring system which rewards skillful play across the board (rather than concentrating high scoring potential in a few arbitrary features at the expense of the rest of the playfield).
6) Flow, intuition, zen, whatever you want to call it: the degree to which the player is able to become one with the game. This is the reason why people play pinball, and it trumps everything else above, in my opinion.
