On a higher level, the success of FG is of course an extension of the success of Panzer General (PG). This brilliant series managed to port the very niche tabletop wargame genre to the PC. These are two very different markets: the average PC gamer was younger and more impatient than the tabletop player, a situation which is probably still the case today (I'll probably talk sociology at a later post). However, without becoming unnecessarily complex, something which turns away many people from the "deeper" tabletop wargames, PG managed to bridge the link and open up the market for the turn-based wargame into the popular gaming crowd.
So how is FG different from its predecessors? The key is selling the fantasy motif and maximizing value from it. Thinking in this direction, it is imperative that the single-player campaign be a strong game, especially given that multiplayer was not as important or convenient then as they are now (also, turn-based games just don't do that well multiplayer). Unlike PG, FG has four heroes in the long single-player campaign, all of whom must be played differently. Krell, for example, can cast a spell per turn, somewhat like Heroes of Might and Magic and has access to magical units. Mordra, probably the hardest hero to play with, summons two random units for each battle and has access to beast units. Also, this fantasy theme lends itself to naturally borrow ideas from the fantasy RPG genre, creating unique hero units and magical items that different units may wield. The most important is probably the use of Shrines, the FG equivalent of the RPG's "treasure chest," which could heighten morale, give a new item, recruit a new unit, summon enemy units, or even change the game mechanic temporarily (such as allowing passage through woods or lava at minimal cost, or decreasing enemy morale). A proportion of the gold is spent on research of new units, which could be allocated at the player's will.
In some sense, while the game is almost entirely tactical and not strategical in scope, it has many of the short-term gains ("goodies") that make Heroes of Might and Magic such a damn addictive classic. However, the more interesting tactics which emerge from the combinatorics of the different unit abilities and combat resolutions (all of which are simple to understand) make FG a deeper game than Heroes . I spend a lot of time planning and have managed to come up with very unorthodox strategies for certain levels. Watching these plans play out perfectly is great fun.
P.S. I played with two "custom" rules: 1) no reloading unless I made a movement error (as opposed to a decision error) or if the situation was totally irreversible; 2) if a unit got to five levels of experience, I would give it a custom name as a "promotion." It was quite an endearing campaign. I would frequently make mistakes, losing sometimes my best unit or hero, but I plowed through. Playing this way, by the final battle I have lost all but a couple of my starting units. This also made the game quite a challenge on the Normal setting, which is preferred as the Difficult setting can be downright unfair.
P.S.S. Very applicable real-life tactics: use concentrated force; flank and avoid being flanked; use timely retreats; use suppression fire to tie down weakened units.
P.S.S.S. As for icing - Fantasy General has a brilliant soundtrack.
(images taken from free-game-downloads.mosw.com and www.csoon.com respectively)