There is considerable variation across different beers, and this means that differences between beers are an important component of the characteristics of those beers. The taste of beer is highly subjective and people will interpret the taste of a different beer differently depending on their own personal perspectives.
While individuals might differ in the way that they view a given beer or what tastes they consider to be more desirable than others, there are some key flavors to look out for that should not be in beer and are an indication that there is a problem in the beer.
In many cases, the presence of off-flavors is an indication of a problem with the processes used to create the beer, but in some cases, negative flavors can arise from factors outside the control of the brewer. One example of this is a skunky odor, which often arises from oxidation as the result of light oxidizing components of the beer. This is one of the reasons that beer tends to be bottled in dark bottles, but it is a problem that is not always avoidable.
Alcohol is a key component of beer, but in some cases it can also be an off-flavor if the alcohol detracts from the overall flavor of the beer. In these cases, the alcohol will often be too harsh. This can occur when the fermentation temperature is too high, as the result of too much yeast, or from yeast spending to long as part of the trub.
So far, I haven't had a beer that has this problem, but I have had several beers that taste “hot”. A beer having too high alcohol content for it's style, or due to needing time to mellow out is not technically an off flavor Fusel alcohol tastes can mellow out over time.
One common off-flavor in beer is often described as fresh cut pumpkin or green apples. This flavor arises from acetaldehyde, which is an intermediate component that is created in the reaction that forms alcohol. The amount of acetaldehyde produced differs across yeast strains, but in general, the presence of an acetaldehyde is not an issue of the yeast chosen, but an indication that more time is needed for the beer to condition.
I have tasted several beers that had this issue. To correct it, depends on what you have done to the beer. If you have already bottled it, the only thing to do is to let it sit in the bottle longer, and hope that the remaining yeast will turn whatever acetaldehyde into alcohol.
If you are still in the fermenter, then adding more yeast or raising the temperature to diacetyl rest conditions can get the yeast more active and help them eat up their left over junk.
Flavors of grass within beer are normally an indication that the ingredients for the beer were stored badly. In the case of malt, poor storage can result in moisture contaminating the grain and the development of musty smells. Additionally, aldehydes will sometimes form within malt, further adding to the grassy flavors. Hops also can be a source of this off-flavor particularly if they are not stored well, or if they are not dried properly before they are stored.
Metallic flavors are almost immediately recognizable within beer, and tend to the result of the containers used to brew the beer, although in some cases, metallic flavors can occur as the result of poorly stored grains. In the case of grains, poor storage means that the lipids in the grain become hydrolyzed creating the flavor.
When metallic flavors arise from metals, this is most commonly because there are unprotected metals that dissolve into the batch of beer. This may occur as the result of cracks in steel pots coated in ceramic. As a general rule, this will not occur with aluminum pots, but can sometimes occur in these pots if the water used for brewing is highly alkaline.
One of my best beers ever was my pumpkin rye. Pretty much all bottles were fantastic, and the flavors only bettered with time. However, one of the last bottles I drunk had a very obvious cardboard flavor. I had always wondered if such a flavor would be detectable (considering my palate isn't so refined), but it was as clear as day. So when you drink one like this, you'll know it.
Cardboard flavor is caused by oxidation of the beer, so I assume it was one of the last bottles I filled, and got some air bubbles in it.
Sour Off Flavors
Most recently I helped a buddy of mind keg a beer of his. I thought the beer tasted a bit sour, but wasn't sure what he was going for. Unfortunately, he didn't taste it til after kegging, and realized there was some kind of infection. It wasn't a repulsive flavor, but it made the beer sour when it wasn't supposed to be. We suspect that it was a strain of Saccharomycetaceae, AKA the same yeast as Brettanomyces, and that it came from the plate chiller or fermenter being contaminated.
Beer that has been light-struck may taste skunky, which is pretty repulsive, and will probably make the beer undrinkable. That being said, I had a batch of beer that basically sat in a light filled room (while in secondary) for about 2 weeks, and I didn't have any problem.
Knowing how your beer shouldn't taste is a great way to figure out how to make it taste better. If you or a buddy create a bad batch of been, keep some on hand to remind yourself what the off flavors actually taste like. It's easy to be overly critical of your own beer and see mistakes where there are not. It also helps to remind yourself WHY you should spend time to brew properly; sanitizing equipment, keeping temperatures within range, and providing a proper storage area while fermenting/carbonating/aging.