The Munchkin case was a travesty. It had significant differences from Pac-Man, both cosmetic and functional. By the same standard, almost all current video games would be illegal, as would most works in any creative medium that fell into an existing genre. But I suspect the court, at the time, didn't appreciate video games as a medium that could have genres. Atari initially lost the case but then won on appeal, apparently to everyone's great surprise.

EDIT: I read the appellate court decision and discovered some things.

First, this only got as far as a preliminary injunction, as far as I can tell. I haven't found anything about an actual trial happening. So, the legal decision was simply that Atari had a decent chance of winning at trial, and the potential harm to them justified an injunction, not that there was definitely any infringement.

The potential for infringement was based not on similar game mechanics, but rather on the more superficial similarities between the characters and aesthetics. In other words, it would be ok to make a maze-eating-chasing game that played just like Pac-Man, but featured squirrels eating nuts and being chased by dogs. It was decided that K.C. Munchkin needlessly copied some small details from Pac-man, like the ghosts and the "gobbler". This was compared to a playwrite copying specific characters and settings rather than re-telling a story in an original way. (That playwrites are not limited to 8x8 pixels and two colors when creating their characters is not touched upon in the decision.)

I found the ruling to be an interesting read which went to lengths to distinguish idea from expression. It even referenced an older case of Atari vs. Amusement World (Asteroids vs. Meteors) which Atari lost:

http://scholar.google.com/scholar_case?case=6441518363892064...

A notable quote from that case:

  It seems clear that defendants based their game on plaintiff's 
  copyrighted game; to put it bluntly, defendants took plaintiff's 
  idea. However, the copyright laws do not prohibit this. Copyright 
  protection is available only for expression of ideas, not for ideas 
  themselves. Defendants used plaintiff's idea and those portions of 
  plaintiff's expression that were inextricably linked to that idea. 
  The remainder of defendants' expression is different from plaintiff's 
  expression. Therefore, the Court finds that defendants' "Meteors" 
  game is not substantially similar to and is not an infringing 
  copy of plaintiff's "Asteroids" game.

The Munchkin ruling also references an older case and quotes:

  Thus, "if the only similarity between plaintiff's and defendant's 
  works is that of the abstract idea, there is an absence of substantial 
  similarity and hence no infringement results."

It later goes on to say:

  Plaintiffs' audiovisual work is primarily an unprotectible game, but 
  unlike the bee pin, to at least a limited extent the particular
  form in which it is expressed (shapes, sizes, colors, sequences,
  arrangements, and sounds) provides something "new or additional 
  over the idea."

This was to be North American's downfall:

  North American not only adopted the same basic characters but also 
  portrayed them in a manner which made K. C. Munchkin appear 
  substantially similar to PAC-MAN. The K. C. Munchkin gobbler 
  has several blatantly similar features, including the relative 
  size and shape of the "body," the V-shaped "mouth," its distinctive 
  gobbling action (with appropriate sounds), and especially the way in 
  which it disappears upon being captured. An examination of the K. C. 
  Munchkin ghost monsters reveals even more significant visual 
  similarities. In size, shape, and manner of movement, they are virtually 
  identical to their PAC-MAN counterparts. K. C. Munchkin's monsters, 
  for example, exhibit the same peculiar "eye" and "leg" movement. 
  Both games, moreover, express the role reversal and "regeneration" 
  process with such great similarity that an ordinary observer 
  could conclude only that North American copied plaintiffs' PAC-MAN.

Had they only copied the idea the case would have gone the way of the Asteroids vs. Meteors case. Where they went wrong is that they copied the expressions used to such a degree that it was found to be infringing their copyright.

Yep. And in the case of Battlezone, a game that is about its appearance, I would argue that the expression can't be separated from the abstract idea. But I doubt we'll ever see a court's opinion on that.

To me, the abstract idea is a 3D wireframe world where you accumulate points and avoid being shot by wireframe enemies, much in the same way that the Munchkin ruling says:

  Other games, such as "Rally-X" (described in Dirkschneider)
  and North American's own "Take the Money and Run," illustrate 
  different ways in which a basic maze-chase game can be expressed.

The idea could still have been a 3D wireframe world where things try and shoot you, but it didn't have to be tanks. Atari's own Star Wars: Empire Strikes Back from 1985 shows how the idea can have a radically different expression.