In the Strategy Gamer's Training Manual (if one existed), the first page should read, "Step One: Play MED2. Repeat.
NOTE: I have just read some of the criticisms of the AI, and there is some merit in the arguments. For example, enemy armies will sometimes just sit there and be wasted by ranged units when they have no ranged units of their own...this seems to happen most often when you sally from behind your fortifications when under siege. Also, the diplomacy mechanisms, unlike the rest of the game, don't always lend themselves to making sense. Alliances, for example, can be tossed away by the AI even when this seems like a bad strategic move. However, while some reviewers have given really low scores because of these issues, I think they are easily fixed...and they don't *always* happen...though they do happen often enough and definitely need patching.
First off, if you've played the previous titles to death and are looking for something completely new, this isn't the game for you. Of course, why look for something completely new when the previous titles were so good to begin with? That's a philosophical debate I'd best leave alone, so here's the essence of the game as I see it:
Trade offs.
Where many strategy games (even the very good Civilization 4) have a fairly simple tipping point where after economies of scale mean you're so far ahead that winning is a foredrawn conclusion, MED2 tries in numerous ways to limit (or challenge) the player's ability to do everything in a turn or even series of turns. Some examples of this come with the fact (a default game option, actually) that you cannot micro-manage your cities/castles unless a general is present. While it's tempting to turn off this feature, its impacts are important. It's no longer a no-brainer to riddle the field with generals, as some of them would best be left at home to tend to the store, so to speak. Here, too, MED2 provides wonderfully rich data to mine in making such decisions -- for example, some generals are horrible leaders and potentially disloyal, but this same general might also be good at boosting the local economy. Perfect candidate for staying home, that guy! Heck, you can even play match-maker and marry specific generals with specific traits to specific princesses in hope of creating just the off-spring you'd like leading your faction several turns down the line!
Unlike Civ4, of course, you can't just plop down cities at will. The map is historical, meaning any cities you add to the empire must be forcibly taken, and your empire's land mass can only come with increasing your troop levels. This is an important trade off that requires balance. It's quite easy in MED2 to be running thin on cash if you're not carefully planning your phases of peaceful building and aggressive expansion. That isn't to say this hard to do necessarily but just that it actually requires a level of strategic thinking that sometimes lacks in this genre.
Speaking of peaceful building, upgrades are EXPENSIVE (a good thing, IMO). Where it's quite easy in most games simply to click here or there to get that fortress in quick order, in MED2, you'd do well to think a few minutes before deciding which city to upgrade and how. That city on your frontier sure could use better (but expensive) defenses...then again, the capitol could become a merchant's paradise with its own upgrade. What to do? Well, you can't always (rarely, actually) do all these things in a single turn, so you have to prioritize. Assess your needs. Even troops don't just magically "heal" - you PAY to replace fallen comrades, so it's best to use them all wisely lest your mistakes bankrupt you.
Cities, too, can also be converted to castles...and vice versa. As you might expect, these are quite different things. A city is better at economy while a castle is better at defense. Each has its unique troops and buildings, too, so this becomes an important choice to make. By the way, converting from one to the other is costly and wipes out certain buildings, so this choice is another great trade off. Along the way, the Pope (or other in-game scripts) will challenge you to do (or not do!) something. Does the request match with your strategy at the moment? If not, is it better to ignore the request? You might be excommunicated, though, or lose out on some free units or cash. While these moments are clearly somewhat artificial and forced in nature, they also infuse the game with some much-needed urgency and yet another layer of strategic thinking.
You'll notice I haven't even mentioned the EXCELLENT real-time battles to be had! I can't recall any other game that manages so much strategic thinking on the main map level before you even get to the issue of actually fighting. Here, too, planning and execution have precisely the kinds of effects that you'd hope they'd have -- having spears ready to take on a cavalry assault actually works. But this is no simple rock-papers-scissors affair that has been done to death elsewhere. There is also morale (affected by such things as flank attacks and having a good leader nearby), honest-to-goodness terrain-related effects (like elevation), spectacular sieges against beautifully rendered fortifications, etc.
Here, too, trade offs come into play: Do you sacrifice your spearmen in a brazen assault on the opposing King's bodyguards (and hope to ruin the enemy's morale by hopefully killing their king)? Or do you sit patiently and let your ranged weapons ... you did bring some, right? ... wear down their forces first? Of course, you might also be getting pelted with arrows, so do you charge in with your own cavalry and try to rout their ranged weapons? Etc. All of this can be paused and planned meticulously. In a nutshell, the outcome is only as good as your strategic ability.
For those who don't like so many restrictions and so much planning (hey, there's nothing wrong with wanting something more light-hearted!), I'd say to stay away from MED2. Rushing through turns and battles will only leave you underwhelmed by the mechanics and the possibilities of what's on offer. However, if you relish the balance between guns and butter, this is your game. If you take great pleasure in watching your Grand Plans unfold in ways that make methodical, strategic sense: Then get this game!