what makes good PCB layout?

Good tools, to start. You must have confidence that your schematic nets match your PCB nets. Place mounting points 1st; and stay away from them with parts/traces. Then place parts -- keep on largest grid you can (100mills). Keep all parts faced same direction, so pick&place doesn't have to spin table too much. When you can't get [all] parts placed a 100mills, go down to 50mills, place some more, then 25mills (no smaller than that).

Don't place stuff too close to edge of board.

If you're over 10MHz, use 4-layers, or you'll never pass EMI.

Traces on bottom should generally run up / down; while traces on top should run left / right (or switched). Point is, you'll get into traps if you don't try to hold to this.

Don't try running 2 traces between IC pins. Never use traces thinner than 6mills; should really try to stay up above 8mills.

Enough for now.

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You try not to have the traces too close to each other and you try to minimize the length of traces.

Remember that parallel traces add capacitance and even short traces act as an inductor.

Fortunately there are many programs that do this for you automatically.

A good layout would have enough room to plug chips in and service the componenets. Also they would be marked well describing the components.

what is PCB?