I have to assume that you are using different types of beer kits here, because you don’t actually say what kind you are making. And there are light beer, dark beer, Pilsner, stout beers and many different brown beers.

Assuming you are using the same type of beer from the same beer kit manufacturer. Here are a few reason that i have found in the past.

The yeast you are using has a flavor content.
The water you are using may be tap water which is loaded with chlorine and other chemicals.
It could also be that you are using a dish soap or bleach to wash your carboys and bottles and not giving them a thorough rinsing.
Someone may be smoking around the production or bottling of your beer.
The glass or jug you pour the beer into before drinking might also be wet, which can cause the beer to be flat.
The sugar used can also be causing a problem in fermentation.
The PH balance needs to be in sync with the recipe on the can.
Storage during primary fermentation should be totally enclosed. Just one fly wil turn your beer into vinegar.
References :
John Taberner
http://www.cooksbestrecipes.com

What you need to do is get recipes for various styles of beer. Ales, Porters, Lagers, Pilsners etc.

You have to experiment a bit and find the ones that you like. Once you change the style of beer it will all taste very different.

For example, a dark Porter is going to turn out very different than a light Pilsner etc. Ales can be made with all kinds of different flavorings etc.
References :
20 year home brewer