There's a few corollaries to the above.  First of all, terrible things that you don't care about generally don't "SUCK."  When you see people on forums ranting and raving about how awful the latest Call of Duty* or Dragon Age* game is, it's because they're deeply invested  in those games in particular and the genres they belong to in general.  You won't see them complaining about, say, the new Barbie Horse Adventures game, even if it's awful.  As the old saying goes, "The opposite of love is not hate, it's apathy."

*Not passing any judgment on those series here. Just noting the perennial phenomenon of strongly held negative opinions.

Secondly, the more you care about something, the more it sucks when it disappoints you.  Furthermore, your emotional investment and expectations give the letdown a tinge of betrayal.  You don't just feel like the experience was bad - it was bad when it should have been and promised to be good.

This isn't a particularly new theory.  The game designer, however, must understand this reaction in his audience if he hopes to control or mitigate it.  The tricky part comes when you design for a genre that is a minefield of conflicting expectations and red-hot passion.  I am, of course, talking about RPG's.

Let's see what each of these points can teach us about player expectations.

1. RPG's touch a raw nerve
   
   

   Them's fightin' words!

   
   The degree of emotional investment RPG fans exhibit strikes me as unique.  We are very protective of our genre and hold lengthy discussions of whether or not a certain game even "counts" as an RPG.
   
   This article's point isn't to decide who's right and who's wrong - everyone has different taste and expectations, and that's okay.  But every fan is a potential customer, and convincing them that our game doesn't suck has a lot to do with managing their expectations.  A game they might have otherwise liked will be dismissed ifs framed in the wrong way, and given the heated passions of RPG fans, if your game is deemed to suck it will suck hard.
   
   I think there's a reason that we argue about RPG's so much - it's the same reason we argue about "art" - we all disagree about what the word even means.
   
   
   

    

2. RPG's lack a centrally defining mechanic
   
   

   

   
    Everyone has a different idea about what makes an RPG an RPG.  Common things include leveling up, story, "role-playing", and exploration.  However, every time someone offers up "RPG's are all about X" someone inevitably answers with, "Are you saying Y, which lacks X, isn't an RPG?"
   
   First, there's experience points and levels.  This can't be the defining characteristic, as plenty of RPG's do without them - Shadowrun and Ultima Online being great examples.  As for story, Dungons & Dragons can be run as a mechanics-only hack n' slash campaign, and procedural games like Nethack have little to no author-imposed narrative whatsoever. 
   
   Surely, role-playing defines "Role-Playing Games," right?  First we have to decide what we mean by "role-playing".  Let's limit the term to exclude things like "playing the role of Mario" in Super Mario Bros.  In this case "role playing" becomes nearly synonymous with "acting in character."  But if acting in character defines "role playing games", then most RPGs are excluded, including D&D campaigns with laid-back Dungeon Masters. 
   
   Finally, let's consider exploration.  Almost all RPG's feature exploration in some sense of the word, but tactical RPG's like Final Fantasy Tactics and Bahamat Lagoon certainly don't, and exploration isn't always front and center in the games that include it - it's often a side dish.
   
   So what, if anything, do all RPG's have in common? Take a look at this chart*:
   
   

   

   
   *This chart is incomplete, simplified, and probably leaves out all your favorite games :)
   
   There's only one feature that all the games on my list have in common - loot.  Equipment.  Stuff.  But does this define RPG's? Surely not - plenty of other games that we don't consider to be "RPGs" have stuff - even swords and armor.
   
   
   Now, let's compare that to this chart for First-Person-Shooters:
   
   
   

   

   
   FPS's are clearly defined by a first-person view and shooting.  It doesn't really matter what else you tack on or what conventions you leave out.  Even Portal can be clearly defined as a first-person shooter, even though your gun is (usually) non-violent and mostly just used for traversing space.
   
   RPG's are not defined by a few central mechanics, whereas most other video game genres are: (Real Time Strategy, Point-and-click Adventure, Tower Defense, 4X Strategy).  Instead,  "RPG" has become a vague term that surrounds certain groupings of mechanics and themes, but without a strictly definable formula.
   
   
   This is by no means a bad thing, nor does it make the term "RPG" entirely subjective.  It does mean, however, that each player walks around with their own personal definition of what defines an "RPG," making it interesting to manage expectations, and dividing the genre into several factions.
   
   
   EDIT: Plenty of people are pointing out in the comments that if you condense loot, skils, and experience into "character growth" then that is indeed shared by all the games on my chart. I can agree with that, but I can still think of examples that break this as a NECESSARY mechanic for the genre. Play a pen-and-paper role playing game purely through role-playing and acting, with no dice rolls or level-ups or skills or anything, and you'll still have what everyone would agree is an "RPG."
   
   Still, I'll concede the point that a central focus on character growth is about as close as it gets.
   
   
   
3. RPG's are divided into clearly defined sub-genres
   
   Perhaps because the mother term "RPG" is so vague, the genre has split into clearly defined sub-groups, each with their own conventions.  JRPG's usually feature strongly authored narratives with parties of colorful characters and turn-based battles.  Western CRPG's generally offer more branching narratives and a tighter focus on the main character who represents "you."  Tactical RPG's feature large parties, tactical battle systems, and narrative interspersed between missions.
   
   

   Avernum 6 vs. Final Fantasy 6

   
   A tension arises between the looseness of the genre itself and the tightness of its sub-genres.  It's difficult to release a new kind of RPG because the game could be pigeon-holed as a member of one of the sub-genres and judged by those standards and expectations, rather than on its own merits.
   
   
   
   
4. RPG's traditionally bill themselves as "EPIC"
   
   Nothing invites both high expectations and harsh criticism like calling your game an "epic experience." The 1997 commercial for Final Fantasy VII is a pretty good example of this kind of marketing.  This is so common that even RPG's that don't bill themselves as "epic" risk being judged by "epic" standards.
   
   

   Gameplay - 60+ hours Story - 30 minutes

    

   This leads to a weird problem.  Most of the time spent in 60+ hour RPG's is filler, and plenty of fans will welcome the opportunity to ditch this crap.  However, if the designer dispenses with all the crap, the game might be considered "too short" since the expectation for an "epic" (ie, long) experience still lingers.
   
   
   
5. RPG's are complex by design
   
   The simpler a game is, the harder it is to call it an RPG.  Although there is no common set of mechanics to all RPG's, there is a common structure - and that's several systems linked together by a meta-game. The nature and depth of these systems varies from title to title.
   
   

   

   
   This results in RPG's having a large number of "core" mechanics.  Furthermore, RPG's have a tendency to add on extra "bonus" systems such as mini-games, crafting systems, etc, to add more "meat" to the game.  Design becomes more difficult as systems increase in number, not only because the designer's attention is divided, but because the number of interactions between systems grows exponentially*.
   
   It's just plain hard to make a good RPG.  I would argue that it's harder to make a good RPG than it is to make a good game of most other genres.  This means, of course, that there's a lot crappy RPG's out there.
   
   *This is why I always tell beginning game design students to avoid designing RPG's - there's simply so much going on at once that there's no hope of success until you have years of experience under your belt and have acquired an understanding of each of these systems in isolation.