In retrospect, the series never really got it right, especially the cornerstone of RPG's, combat, is always the element that feels the most underdeveloped in the games. The skills are a nice addition to the game, but on the other hand, they're not strictly necessary element either, even if you think about creating puzzles with different kind of solutions when playing with different classes. Maniac Mansion for an example managed to offer similar kind of gameplay with just having a pool of characters with different skills, which leads to solving the game in a bit different manner, as not everyone can fix a telephone for an example or record an awesome demo tape.
Also, especially in the first 4 games, the RPG elements effect relatively little on the gameplay itself. Sure, a thief can't use heavy weapons, but there are no penalties for using heavy armor, the amount of equipment you can gather is extremely limited, especially when you compare QFG-series to games like SSI's gold box Dungeons and Dragons license games and so on.
The three character classes, a magic user, a thief and a fighter, do provide some amount or replayability to the games, especially when you compare them to your typical adventure game. Each game has things only a character with certain skills can do, so even if you are playing with a hybrid character it is a nice idea to play the game with a pure character class as well, especially when you get to the third game. Both 4th and 5th have things only a specific class can reach or see, so all that adds to the longevity of the games.Then there are the different puzzle solutions for different classes, so that adds to the game as well. But again, a character with skills other than his own can solve puzzles in all the ways possible, the difference is, that you only score when solving in class appropriate means. That matters only if you're interested in maximum scores though.
The thing is, that after you've played the games through once, they don't really offer a huge amount of challenge for additional playthroughs. Especially if you figure out how the skill systems work, it is fairly simple to just max out the character very quickly. And the adventure parts themselves don't change, so if you know answers to the puzzles, there's not really that much meat in the games to make them last.
I'd even go as far as to claim, that as RPG games Quest for Glory fails fairly miserably. The RPG elements the series has always feel more like something that was just tacked on in order to pad the length of them rather than something that is strictly necessary for the big picture. While it is nice to have the feeling of continuity after importing your character from the previous game, in hindsight it also feels unnecessary, as there's the same feeling of continuity in every Larry or Space Quest game when you see the familiar character enter the room.
Where the series succeeds very nicely at is the world building as well as giving the feeling of an adventure. These elements alone are more than enough to cover the more lackluster areas of the games. While each game is its own entity, they also do refer to future installments, by providing little snippets of information about what's ahead in the series. Unlike many other RPG's, Quest for Glory isn't happy just having one type of setting it dwells in, but Lori and Corey Cole bring in elements from all kinds of settings, providing gradually fleshed out the world where the hero of the series exists in.
Interestingly enough the series was supposed to have only 4 games in it at first. The third game, Wages of War, was done only because the designers thought their initial idea of jumping directly to more darker Mordavia from Shapeir would have been a too sudden change for the series, so they made the Africa themed mid-adventure instead.
This jump is an interesting, albeit very flawed one, as Wages of War isn't very strong or particularly well-designed game. It feels empty and overall it also feels very short, which it is, especially if you consider how little things it has to do with different character classes. Though perhaps Wages of War also is a necessary evil, as maybe because of it Shadow of Darkness stands out so much from the rest of the games.
As I said, I do think Quest for Glory was probably the most ambitious game Sierra did produce back in the day. I'd say they were ambitious even on contrast on other game companies, even on those that were producing straight RPG's. They tried to bind together two different genres and in some of that they succeeded and in some, they failed.
This comes out very clearly with the first two games. QFG1 is an especially simple game, and if you know what to do it's not a particularly difficult task to play it through in an hour or two and that even leaves you amble time to max out some of the skills. QFG2, on the other hand, feels very complex game on the first playthrough, but after that it's just far too glaring how much the game itself depends on you to just aimlessly wander the deserts fighting the enemies and maxing out the skills, as a good bulk of the puzzles themselves are tied on specific in-game dates, so you need to spend the time between the plot points doing something.
While I like the series as such, I do need to admit that as a whole the games haven't aged particularly well. The things I thought would be an asset to it, particularly the RPG elements, feel too much like tacked on elements, despite the games were designed to be RPG's from the get go.
At times I've been wondering why there's been relatively few RPG/adventure hybrids, but after playing through the QFG series, I've come to a conclusion that as such it's not a particularly easy genre to design games for, especially if you want to combine adventure like puzzle solving in it.
The adventure game elements themselves, while a good addition to an RPG game, tend to end up as being too simple, as they themselves have no real element of surprise after you've solved them once. The RPG elements can end up feeling tacked on padding mechanic, especially if the adventure itself feels too simple. There's a lot of pitfalls in a design like this and in my opinion, the Coles succeeded only partly in their games and not every time in all of them.
In the end, I do think the games could have endured better had they been more strongly geared towards RPG genre. That might have lead the game design been driven with the terms of the RPG side rather than the adventure side, which feels, thanks to the Sierra engine, much more dominant.
If you are interested seeing what I thought about the individual games in the series here's links to relevant entries:
Quest for Glory 1
Quest for Glory 2
Quest for Glory 3
Quest for Glory 4
Quest for Glory 5
The series itself can be bought from GOG as a collection containing all the games in the series. It's not really bad value for money.