Story aside, Gothic 1 is a definite must-play. The game is great in many ways but as Roxor said, it makes Gothic 2 even greater because you get to see the past game's influence and effects all over -- plus if you are going to enjoy one of the games then you will enjoy the other game too, why not get more of a good thing? The beginning to g1 is great too - you're this useless weak piece of shit who gets bullied by people left and right and can get his shit fucked up by a wolf. hell, you don't even know how to hold a goddamn sword past "not on the sharp part". the slow rise to a position of worth is something that is honestly rewarding, and it feels awesome to finally be given a guild's armor and be told "you're one of us now" - g2 has these moments as well... but when you start the game by having a powerful necromancer summon you and gives your first quest as "go save the world", it just lacks that initial "where am I? what's going on? why is everyone so mean to me ;(?"
edit: yeah the brohemian rhapsody is pretty righteous too - in g2 you know who Gorn is from people telling you, but you might feel no need to help his ass out - in g1 the circle of bros really help you out and make you feel like they're actually worth something as NPCs/characters giving you much more reason to get involved with them again.
Also, disregard that bullshit Nomask said about the controls - the new mouse based controls in G2 are great and I would love for a mod that implements that into G1. The original controls leave you somewhat stranded in finger use and button mapping - it gets easy enough to use as you get used to it, but it still means that you can't strafe/hit as easily (like you would need to if going up against a Troll).
G2 with NOTR is HARD, but at the same time totally worth it as the game is incredibly better with the add-in. There are some really great quests added in, and many things become much more interesting and challenging.
I will say this tho - playing a fire magician will be very challenging as you will be strapped for LP, much more so than a mercenary.
Big important tip: disregard the tip that trainers give you about raising your weapon skills together - that no longer applies in NotR, so you can just focus on whatever combat skill you want to increase and disregard the other ones. In vanilla g2 you had to increase both weapon skills of the same category together; this is no longer the case.
edit 2: also get this mod for g1 - it installs g2's much higher resolution textures that are actually very pretty over g1's lower resolution textures but changes nothing else. definitely worth it and a very small download:
http://www.worldofgothic.com/dl/?go=dlfile&fileid=121