Ok, I think I can help here and am going to throw a whole bunch of information at the wall:
In addition to being a professional brewer who has worked at several breweries producing both all grain and extract beers I also was briefly the manager of a local home brew store that sold alot brewer's best kits. The key with those kits (and brewing in general) is freshness of ingredients. 9 out of 10 kits on a shelf have been there for several months, in addition to sitting in a warehouse for several months.
Always do the following inspections prior to purchasing (beer kits are considered "food items" and most shops will not allow returns):
Check the yeast expiration date- This right here will give you an idea of how old the kit is. If the yeast is expired, or really any older then 12-16 months, bring that to the attention of the shop. A good shop will swap the yeast for a fresh pack for free. If the shop does not... don't shop there! The yeast expiration date will also give you some idea of how old the ingredients as a whole are.
Check the extract date- Different manufacturers of malt extract have different coding systems, the most common being Briess or Muntons. Both explain their date coding on their website. This is (arguably) the most important part of brewing a good beer from a kit. If the extract is expired bring it to the shop's attention. A good shop will either swap the extract for new(er) extract or, atleast, give you a discount in the range of 20-30%. If the shop does not... don't shop there!
Check all bags/packaging for tears/holes- Make sure the hops, yeast, grains, dme bags, etc are not punctured/ripped/etc. If they are, bring it to the shop's attention. The should swap or discount, if they don't... don't shop there!
After you go through all of that and decide to make a purchase of a kit you can drastically improve the final quality of your beer by doing any/all of the following (regardless of what the kit instructions say):
Full volume boil- boiling a small amount then topping up with water can throw of your flavor profile and color
Boil longer- if you are doing a full volume boil you should be boiling for atleast 45 mins, 60 is even better, longer (70+) is best.
Pitch more yeast- if you use a pitch rate calculator you'll find that the 1 pack of dry yeast you typically get with a beer kit is not enough viable cells to ferment 5+ gallons of wort that is 1.040+ so you'll always have more complete fermentation and cleaner overall flavor profile by adding a 2nd pack of yeast. It's very much worth the extra $2-$6, the shop may even give you a discount if you explain why you are getting an extra pack.
*Always skip the secondary "fermentation"- Your beer can sit on the yeast cake, autolysis is a myth that has been debunked by many commercial breweries and legit labs. Implementing a secondary requires a transfer that will usually introduce oxygen to your beer...o2 is BAD at any point after your fermentation has started. In order to properly transfer to complete a secondary fermentation at home without introducing o2 requires a c02 tank and purging everything with c02 prior to use.
*---->A few exceptions to the above rule: you can use a secondary if you are aging beer, souring, adding oak, adding fruit, dry hopping, etc. Still, I advise against it. Any of those "additives" can be added to your regular primary fermentation.
Add irish moss/whirlfloc, if the kit does not already contain it- this is a cheap and effective way of clearing up your beer before it even hits your carboy/bucket.
Let your bottles condition longer- When you pop open a bottle to check the carbonation and think "yup, it's ready!" give it one more week. It never hurts. If you made sure to not bottle prematurely and not overprime your sugar then your are fine.
With all of that being said... I would suggest you buy a kit from Northern Brewer or Austin Home Brew over a brewer's best kit any day. The NB and Austin kits are put together by them with fresh ingredients they get from their suppliers. The Brewer's best kits are put together by the brewer's best company then sit in their warehouse, before going to a supplier and it sitting in their warehouse, before going to a lhbs and it sitting on their shelf. Even the "freshest" BB kit right off the truck into a lhbs is still anywhere from 6 or more months old... when it's "fresh"
Hope this helps!
One last thing, when you have a few minutes to kill hop over to the Briess website and look through their recipe database. They have a ton of great recipes and you can literally just print then take to your lhbs and say "I want this stuff". I exclusively use Briess malting products at my brewery and they are by far the best I've ever worked with in terms of customer service, freshness, and overall quality of ingredients.