The light brown streaks are normal; at least they are for every one of my batches.
Regarding infections, there are 2 general types: bacteria and wild yeast. Bacterial infections will generally produce acids which will sour the beer. There are bacteria strains that will produce cooked vegetable aromas/flavours, strains that produce diacetyl (butterscotch), and other unusual things. Wild yeasts will produce phenols. Things like cloves, bandaids, plastic, nail polish remover, smoke, spice/pepper, etc. Under unusual conditions, normal beer yeast will also produce some pretty bad phenols. These conditions are very poor yeast health, underpitching, and insufficient (almost nonexistent) oxygenation. Very high fermentation temperatures will also do the trick.
Unfortunately there is no real way of detecting an infected brew by looks alone, other than having a look under a microscope. The homebrewing literature states that a ring around the neck of a bottle at the surface of the beer is a sign, but I'm not 100% convinced of this. I've had homebrews with rings that showed no signs of infection, but perhaps the infection was indeed there but hadn't gained a foothold yet.
This document describes a lot of beer flavours/aromas, and their causes.
