I am asked this question on a regular basis. Part of the answer is subjective, what appeals to one person does not necessarily appeal to someone else; the old adage of one man's trash is another man's treasure holds true here. There are, however, some common characteristics that most of the sets considered great by Lego enthusiasts share. These are the same characteristics that I try to use in my reviews to evaluate a set's merits and demerits. They are as follows:
As I have said in many reviews, playability out trumps all. A set may have horrid pieces, be too expensive and be ugly as can be, but if it has great playability I will forgive it almost anything else. The reason for this is that ultimately Legos are meant to be played with and if they meet that requirement then they have fulfilled their mission. Playability is defined as how simple it is for a set to spawn original play. A set with good playability will require almost no imagination to use and keep using. A set with poor playability will be difficult to play with after a month or so because all of the options for stories that are easy to imagine it being a part of will have been exhausted. A set that still spawns original play a year after it's purchase has good playability.
One of sets with the greatest playability of all time (in my opinion) is 1996’s Fort Legoredo. Its size, detail and setting make it a natural for hours and hours of play. I quite literally played with this set for hours a week for several years.
A recent set that serves as an example of poor playability is 8092 Landspeeder from 2010. After you re-create the famous “These are not the droids your are looking for scene” exactly what are you going to do with this unarmed, non-flying, people mover?
Great as a collector’s item but in terms of a set to play with, not so much.
After playability comes the pieces. In much the same way that a set with bad parts can be excused if it has great playability so a set with mediocre playability can be forgiven if it has great parts. Inevitably sets get disassembled (some kids never put the sets together at all!) and when they do you want the part selection to be good so that original creations can be made with ease. Good sets have pieces that are small to medium in size (modular pieces such as specialized roofs are bad because they can only be used for their intended purpose and not for anything else) in primarily basic colors (black, white, gray, yellow, blue, brown, tan, green or red). Sets with primarily modular pieces or funky colors (orange, purple, lime green, or beige) should be avoided because they will be very limiting when they are incorporated into original creations.
An example from this year of a set with great pieces is 7066 Earth Defense HQ. All the pieces on this set are small (no modular pieces), in excellent basic colors and are all very useful.
A set with an example of horrible pieces is 2006’s Piraka Stronghold. This set is from the short lived (thankfully!) Bionicle playsets theme.
Exactly what are you going to do with all those specialized pieces once you get bored with this set? Sure that big mask piece is cool but what would you use it for?
Crossover is an often overlooked characteristic that can be very important. Crossover refers to a sets ability to appear in stories set in different themes. My most often used example of a set with good crossover is a medieval blacksmith's shop. That set can be used in its intended setting (castle) but will also look right at home in a dock town being raided by pirates or as part of a historical district in a modern city. This ability to easily jump from theme to theme will add substantial play hours to a set thereby increasing it's value.
One of the sets with the most incredible crossover that I have seen is 10193 Medieval Market Village.
This set would fit in almost any of Lego’s themes. It could be a harbor town for pirates to raid, a historical district in a city, it could even be a technologically backward settlement in a space setting.
I put less emphasis on this than other collectors for the following reason, some pieces take more plastic to produce and they are worth it (boat hulls and castle walls are good examples). Likewise, Lego will sometimes try to hide the fact that a set is not that good by inundating it with small pieces to boost the piece count. A recent example of this is 4645 Harbor which is significantly smaller and less complete than any previous harbor offering. This seems counterintuitive since the price and piece count are roughly similar until you see what Lego did which was boost the piece count by making the harbor a grain harbor and including a bunch of small cylinders to represent grain.
Each of those pieces boosts the count while adding nothing of substance to the set. In the reverse some sets have smaller piece counts but they are really good pieces which make up for it. So I would recommend that you be conscious of the piece count (the rule of thumb is that a set with 10 pieces for every dollar is a good value) but don't rely on it.
Last but certainly not least is price. This one is pretty self explanatory, some sets are just not good value. More than any of the other characteristics this one is a judgement call because certain sets are worth more to one person than another. If you want the set and are willing to pay what Lego is asking then go for it. Trust your gut, I have bought several sets that I didn't have a good feeling about as it related to the value I thought I would get for how much I was paying, and that gut feeling has never been wrong; I have always been disappointed. The biggest example of this for me was 6991 Monorail Transport Base from 1994.
This set is captivating due to its size and uniqueness, but it is actually cumbersome to play with and does not come with enough track to really make anything cool with it. Conversely I have bought sets that have been critically hated by other enthusiasts that I had a good feeling about and have never been disappointed. One of the main examples of this was 6975 Alien Avenger from 1997.
This set breaks a lot of the rules (big pieces, low piece count, not the greatest colors) but it was a blast to play with.
There are exceptions to all of these but as general rules of thumb these characteristics can serve as excellent guides as you navigate the numerous offerings Lego has out there.
The Lego Chronicler