As a general rule of thumb in not just Lego Investing, but in collecting in general, in terms of what will appreciate in value, is that if it displays well, it will do well.
In my 34 years of collecting that is ultimately what I have learned. As a child, I collected anything and everything of any theme. I was a huge trekkie in the 1990's and that was a total disaster, however, the only items that retained some value, and even potentially appreciated were the ships. Why did the ships appreciate, and not the endless amounts of action figures. Because it is DISPLAYABLE.
People want to pay money for things they can display. Many people buy things to have a complete collection, but ultimately, the items that ALWAYS sell for the most are the ones that are the most displayable. My gobs of action figures Mint On Card don't matter because you can't put them up in an office and have them be acceptable from a display standpoint without looking bad or cheesy. Some of the nicer action figures these days do pretty well, but again, it must be displayable, not look cheap, and typically the more articulation points the better. But again IT IS DISPLAYABLE.
I always use the CEO Rule. If you can imagine this in a CEO's office on a shelf or in a glass display case, then it will likely do well. If you can't it is not as good of a bet. The Lego playsets almost ALWAYS do worse than the vehicles and ships simply because most adults, who are the people can afford to pay 1,000's for a set, do not want to display Arkham Asylum or a Hoth Attack playset in their office. The Modulars do well because they are a series, and many people want to have a whole town, and to have a complete town, they have to buy the first ones, which no one stock piled at the time. As time goes on, people jump on in the middle and then need to buy the subsequent versions.
Even a weak IP (intellectual property) or total lack of IP (Think Taj Mahal, Eiffel Tower) will do well if it can be displayed. While a strong IP like LOTR and Harry Potter are not a bad thing, There were very few truly ICONIC sets made from those IP's. If there was a UCS Hogwarts, that would have had major potential, but sadly none was ever made. And while there is the ToO, it has not done well in retirement, again, because it is not something I would put in a CEO's office. It has a Goth feel to it, and a business Exec does not want to convey a "Goth" feel. However, Star Wars has a more "intellectual" feel about the property, and most of the vehicles are instantly more recognizable than the ToO.
Lastly, an appreciating collectible CANNOT BE A FAD. No matter how displayable something is, if it is a FAD then EVERYONE and their mother will buy one. (Think Beanie Babies, Furbies, etc.) With a Fad eventually the market will shrink as the Fad dies. As with anything, pricing is based on supply and demand. When a fad dies, the supply is massive, and the demand miniscule causing the implosion in prices. Why is my Star Trek Collection worthless, cuz it was a fad. Why are my beanie babies worthless? Cuz it was a fad. While Franklin mint items are beautiful to display, they were again Fads, and people bought them in droves as they believed they would be worth something some day.
Legos are NOT a fad. Even though Legovesting is more popular now, it is still not a FAD. There are not groves of people lining up outside stores so they can invest in Legos. People do not call every store in the area so they can buy legos. Not only are Lego's not a fad, but the number of Lego fans grow worldwide EVERY YEAR. Lego's are universal and loved in every country.
Legos are displayable AND socially acceptable to have in your office. I have many toys and Lego's in my office and most people find them amusing and entertaining. The toys I keep in my office that are not legos are typically very well made Sci-Fi ships with nice display value such as the Art Asylum Star Trek Series, Bandai Star Trek Model Series, and Battlestar Galactica Moebius ships. The business executive professional market increases the market value tremendously, and is what allows the toys to sell for hundreds and thousands. Children are NOT likely to spend 2000 on a MF or Taj Mahal. Adults with expendable income will though. This is why the bigger the set, the more people are willing to spend for it. Most people willing to spend 3000 on a Taj Mahal do not want Minifig clone armies displayed in their office or their house. The professional DEFINITELY DOES NOT want Chima, Lone Ranger, Nexo Knights, Friends, or Prince of Persia in their office either. They wouldn't mind a UCS Imperial Shuttle or UCS Slave I though. While some of the sets might return a nice CAGR such as Friends, they will never hit the astronomical heights of the UCS Sets, simply because children do not have the cash to pay 1000 for a friends set.
Lego's are like the Apple of the toy world, and as such are able to command higher prices than the average toy.