Although both Half-Life 2 and Portal have similar gameplay, (solve puzzles in a 3-D environment), Portal is vastly superior to its larger brother.
1) Half-Life 2 is a liar. It is dressed up as an action FPS, but it is a series of abstract puzzles occasionally interrupted with shooting brain dead soldiers. Portal plainly calls itself a puzzler, and has depth (both puzzle-wise and plot-wise) beyond most puzzle games.
2) Portal has no boat segment.
3) Portal is funny.
4) While HL2's detailed environments are great for setting the mood, they are positively cruel when the game makes you find some ladder or item to solve the current puzzle. Portal's much simpler look makes solving the puzzle a test of brainpower, not "until you find that ladder in that alcove, there's no where you can go."
5) When you're railroaded in Portal, it's expected. It's a series of aptitude tests. It makes sense that there's only one way to go. In HL2, you're in a vast cityscape, and there's still only one way you can go.
It's a wonder Gordon's foes didn't place a waist-high wall across his path somewhere. Without items to make a little ramp, he'd be unable to scale the barrier slightly higher than his pathetic jump height.
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