Organized as effectively as I could manage on a coffee break. If you have any questions or need some clarification\expansion, just ask.
What's familiar:
1. Weapons. Bolt guns, lasguns, pistols, chainswords, autoguns, knives, grenades, etc. Lots of options, lots of gear, all of which will be familiar to you or at least tie in with the theme and flair of the
40k world. Some of the more exotic items are rather unique, but all in all, they just represent special versions of familiar things.
2. Models. Still using 28mm models, still using round bases. WYSIWIG is fairly important, and adds more to the game than in
40k (I'll expand on that later).
3. Line of sight. When you shoot, throw, chuck, fall, dodge, or do any other kind of action, you measure and become the model to see how it plays out. If you can see the guy, you can hit him (cover does play a role, discussed later). Expect there to be more dialogue and "mutual agreement" between you and your opponent.
4. Stat lines.
WS,
BS, M, A, W, etc. All of them represent the same thing in Nec as they do in
40k. So if you have WS7 and BS2, you suck at shooting and kick ass with a sword.
5. Testing. Still take
Ld tests on 2 dice, characteristic tests are equal to or less than, etc.
Now, while there is plenty that is different, I'll only go into the biggest things:
1. Campaign driven. This is a dynamic game. When you play
40k, you agree on a points limit, go online, look up what everyone says is the best thing to put in a list, then go play that, have no fun, and try something else the next time (yes, im being sarcastic, roll with me a minute here). The game, regardless of outcome, has no bearing on the next. Nothing ever gets better. Draigo is Draigo, a guardsman has a lasgun and hits on 4's, all space marines have frag and krak grenades the moment they're enlisted, etc. Necromunda will have you start with 1000 credits to create a gang, and you go from there. Every game there after, your gang will level up, gain new skills and abilities, better (or worse) stats, you can buy new equipment, switch gear around, and grow to truly amazing levels of power and prestige.
2. Shooting and combat. After a few games, you'll have enough stat boosts or items to make shooting easier, but to start out it's fairly difficult. Shooting is dynamic, meaning you rarely hit on what your stat would dictate. There are no cover saves in the game. Being in cover makes it harder for your opponent to hit you (gee, what a concept...). Less than half the model covered, -1 to hit. More than half the model, -2. Weapons will have bonus's or penalties to hit as well. Shooting with a pistol? Probably at least a +1 at short range. Is that a hunting rifle with a powered scope? Awesome, -1 at short range (no Halo sniper shotguns here). Bottom line is that shooting will be familiar, but is definitely different than
40k.
Combat is easier early on. The main take-away is that when you charge in to fight someone, only 1 of you will (typically) actually hit the other. You'll roll for a combat score, and the winner will get however many hits his score beat your score by. Ties, parrying, special skills, fear, and myriad other things come into play, but rest assured it is not a simple matter of "ok, so lets do this combat, what's your initiative?". It's way more entertaining
4. Armor. Unless you play as Adeptus Arbites (oh no, its the !@#$ing cops), it's really more like "Um, what armor?". No one has it by default. It can be acquired during your games, but typically only a 5+ or 6+. Some specialty gangs will have awesome armor, mainly Spyrers, but thats a special case.
5. Gangs.
40k has different races, Nec has different gangs. The majority follow the same rules, but like
40k some are better suited to shooting, some to combat, some are all rounders, etc. As always, find the background and models that suit your aesthetics, and go from there.
6. Terrain. You're playing in The Hive. You'll need to construct or obtain multi-level structures. Catwalks, gangways, substructures, chemical dumps, sketchy side ladders to climb up levels, etc. Search around for "necromunda terrain" and you'll get an idea of what I'm talking about. You would rarely play a game on a flat "
40k" terrain setup (trees, a hill, some ruins, an overpriced landing pad which was added into the ruleset in a not-so-subtle way so we could increase our pathetic terrain sales, etc.). It's actually apart of most of the scenarios you'll fight to spend the pre-game alternating with your opponent while you two construct The Hive.
7. Last point I'm going to bring up is the general "overall feel" of Nec vs. a
40k game. If I play a space marine army in
40k, and have 30
tac marines in 3 squads of 10 with the usual equipment, in rhinos, with 5 termies in a land raider crusader, two flying goldfish, and marneus calgar chilling with the termies. Thats a total of 35 infantry with various weapons, 6 vehicles, and 1 "character". I'll only be able to dictate what the unit does as a whole, though. I must keep them in their units, and a single marine will rarely make a difference.
In a Nec game, for a first game, this could be a standard setup: 1 gang leader (hand flamer, sword), 1 heavy with a heavy stubber, 1 heavy with a flamer, 5 gangers with a mix of autoguns and lasguns, and 4 juves armed with stub guns. That's 12 models. 12. Barely bigger than a single squad from the above
40k game. Yet I'll get to move, shoot, assault, and act 12 times, every round, and every single member of my gang matters (ok, well not the juves, !@#$ them till they prove themselves, they're freaking running straight in to cause some hell or take the first bullet). There is no coherency, you can be as far or as close as you'd like. There is no shared unit actions, you can shoot with one guy and charge with the guy standing right next to him, doesn't matter.
Definitely give the basic rules a read-through, and check your area for a
FLGS that has some players knowledgeable or willing to play a campaign with ya. If you find you're still interested, hit up this site for some great info and updated ruleset:
www.yakromunda.com
Best of luck in The Hive
10,000 
if i ever game again then i wouldn't mind a few games of necro again (i say if i ever game again is because everyone [i know] is busy playing