Wiki is down today, so I wouldn't mind reading up about it from you Fnord.
Is it just a team-based shooter, or is there a lot more to it ?
There is a lot more to it actually!
Jagged alliance 2 is, at its heart, a TBT (turnbased tactics) with CRPG elements.
The game starts rather simple, you are told that the fictional country of Arulco is in need of help, and that the tyrannical queen Dedriana is making her people suffer, and that it is your job to save the country. You are given a starting sum of money that will be used to hire other mercenaries. You are also allowed to design one mercenary of your own. He/she is not a representation of you though, he/she can die and it won't end the game, but as this is the only mercenary who you are allowed to design yourself, it is in your best interest to keep him/her alive. At first you are asked a few personal questions, which will impact what special abilities your mercenary will have (I read up a bit on it after I had created my character, and there is a relatively wide range of special abilities, ranging from being better at certain weapon types, to being better in night fights, to just being better at talking to people. There are negative traits as well, like for an example one will make your team become less happy, as you are such a jerk to them). The game never tells you straight out what these do, you will just have to guess if there is any specific trait that you are looking for (I found the night fight trait to be invaluable). After that there is some stat/skill allocation to be done, nothing too complex, but enough to warrant multiple playthroughs with different characters. After this semi-you is done, you are allowed to hire a team of mercenaries, all of which come with their own skills, abilities, equipment (you can chose to not let them come with any equipment, which will make them a bit cheaper, but for the starting party, you really want everyone to have at least a gun), likes & dislikes. Each mercenary can be hired for either a day, a week or two weeks, and the longer you hire them, the lower the price per day will be.
After that, the game itself starts. You are dropped right into an enemy occupied village, and you have to kill all the enemies in the area. The game plays out in real time, as long as there are no known enemies in the area, but as soon as you see an enemy, the game will switch over to a turnbased mode. And this is really the meat of the game, the tactical gameplay. Much like in X-com, your soldiers have action points that they spend in order to walk, shoot, use their equipment, reload their guns and so on, and also much like in X-com, your soldiers are quite fragile. Not quite as fragile as their X-com counterparts, but if you leave a soldier out in the open, chances are that he or she will get shot to pieces and die, so it is important to always make sure that your opponent has a low hit chance against you, either by standing behind cover, going prone and/or try to engage them at a longer range than their weapons are made for. It is also incredibly important to not get flanked, as that can result in a lot of incoming fire against a single one of your soldiers and your cover won't help you.
And because your soldiers are relatively fragile, fighting in low-light conditions is preferred, as it means that you will take less incoming fire. It will of course also mean that you can't see as far, but this is where the night fighting trait comes in, and you can also acquire night vision goggles, which are immensely useful. Enemies tend to gather around lightsources (around buildings, near boom barriers outside bases and such), so you can, if you play smart, take advantage of this. It is important to realize that the enemies can hear you (and you can hear them), so gunfire, running or other noisy activities will attract more enemies. This is another thing that can be used to your advantage, as you can set up ambushes.
There is a surprising depth to the tactical elements of this game, something that is not immediately apparent, and the game really rewards good use of all the elements. It actually does a far better job at this than any other game in this genre that I have ever played. I just feel that it could have introduced these elements a bit better, for an example had the first fight been during the night, then the use of sound and low light conditions could have been introduced to the player, instead it is up to the player to discover how to use these elements.
There is also a strategic element to the game. Once you are done with the first village, the game world opens up, and you are told that you need to conquer as many town as you can, as these generate income for you. Your mercenaries won't be happy if you can't pay them, so getting at least a few of these towns is important. Every town apart from one is heavily guarded, but once you have managed to take them, the people will be happy to provide some financial aid (all income is generated from mines, which are part of the towns). Of course, if you leave a town undefended, then queen Dedriana will just send her soldiers to retake them. In order to prevent this, you need to train a militia, which will cost both time and money, but the investment is usually well worth it. You can also hire more mercenaries as you go along, and while you can't have more than 6 in each team, you can have multiple teams, all working independently. While the strategic element is not nearly as deep as the tactical element, it still adds to the game, it makes the game world feel more coherent and alive.
The game also has a relatively simple RPG element to the game. The soldiers gets better at what they do (if one uses his/her gun a lot, he/she will get more accurate, if one works with maintaining the equipment, he/she will become a better mechanic). Apart from that, there are some side quests that can be solved through dialogues, some people who will join your cause if you can convince them to and a fare amount of dialogue to be found in the game that is just there to add to the immersion. The different mercenaries also have opinions of their own, and while I never experienced that my actions had much of an impact on it, the presence of other mercenaries will affect their mood, which in turn affects their loyalty. Simple, but it does give them personalities of their own. Also, every line of dialogue is fully voiced, and considering the amount of mercenaries you have at your disposal, that is a lot of recorded dialogue, at least compared to what most other games at that time had. Of course, with that many mercs, they don't have a whole lot to say.
Overall, I feel that the game could have done a bit more to introduce its different concepts, but once I got into it, I could see why this game is praised by almost every single person who has played it. This is a deep and very interesting game, which seem to offer a lot of replay value. It has a few technical issues (it crashed a few times on me, and my savegames got corrupted twice (after first time, I started to save often, and in different slots)), its graphics were never considered to be very good, but even with those flaws, I would recommend that everyone who has not yet played this game does so. Make sure that you get the Gold version, it has a few bug-fixes and it is also more new computer-friendly (the version on GOG is the Gold version). Getting the game is not very hard either, even though it was released in 1999, new boxed copies are still being made, and it is also being sold on GOG, Steam and Gamer's Gate (boxed copy seem to be the cheapest way to get it, followed by GOG (for us Europeans, but the Gamer's Gate version is currently 50% off)).
*edit* Oh, and I will make sure to check out some of the mods. I've heard a lot of good about 1.13. Thanks for the link!