I wrote this to a friend of mine that asked the same question. SORRY it's LONG!!! 
To put things in perspective I was a 10th Prestige level 65 in WaW and a 5th Prestige 55 in CoD4 (stopped prestiging). So I have "some" experience here....
Hawk,
To preface this reply - as I type my eyes are bleeding from pain as I have logged over 30 hours of play time since getting my hands on the game.
In short - it's badass. So what makes it badass? Well - Think CoD4 with better graphics - larger maps - more weapon variations - cooler perks - cooler kill streaks - riot shields for protection classes - and that's Modern Warfare 2.
These are some differences that I personally feel stand out:
PROS:
1) Host Migration - Of course you knew this was going to be part of the game but as you see how often it happens it reminds you of the times games ended due to the host leaving and was thoroughly annoyed.
2) Larger Maps - This can be a good or bad thing - it's bad until you get to know the maps cuz there is so much shit and so many places to hide. I think I'm finally appreciating their size a bit more - It gives a lot of variation for each one and different game types.
3) Slightly More Custom Class Variation. This is a cool one. When you create a class (only 5 custom classes that I know of - Not sure if prestige changes that) you get a few different options.
Primary Weapon (Assault, sniper, riot shield - shield replaces main weapon, etc)
Secondary Weapon - Can be a shotgun, sub machine gun (machine pistol) etc - Much cooler 2ndary weapon.
Equipment - This is like the new one - This is your grenade slot that you can swap out for different things. Throwing knives, tactical insertion, blast shield (less explosive damage), etc. These new things are cool to try - so far though I'm hard pressed to get rid of my frag grenade. I am going to start using the sticky grenade though to try that out.
Special Grenade Slot: Flash, Stun, Smoke
4) Perks. New Perks are very cool. As you use them you get a new upgrade called "Pro" version of that perk. These are somewhat minor though.
1st Perk: Sleight of Hand is badass and will become your 1st perk hands down - but you can also choose Scavenger - that lets you pick up bullets off any corpse - they work for your gun no matter what you pick up. Very important for some weapons but sucks to give up sleight of hand for it. (Pro sleight is faster looking into the sights)
Marathon is also a good choice for unlimited sprint - awesome for CTF games.
Don't forget BLING - multiple attachments are awesome but I only find myself using it for my sniper rifle (Heart beat sensor and Thermal Scope
- which is BAD ASS)
2nd Perk: Stopping Power - I've found it hard to swap this out for anything - once again. Stopping power is simply the best perk hands down for no. 2. Swap this for faster movement, uav protection, etc if you desire. The ONE perk I had in there over stopping power was Hard Line. Basially gives you your kill streak bonus 1 kill early. Very nice to rack up kills faster.
3rd Perk: You will use steady aim most of the time. (Pro version is holding breath longer while sniping). Additionally a new one called Scrambler is worth trying out. It scrambles the enemy teams radar - but also tips them off that you are near.... I think if 1 person on our team had it on - that would be enough.
Also, as of just hitting level 40 - I got "Last Stand" for Perk 3. Last stand is awesome now - you can crawl while you're hurt AND use your sub machine gun pistol..... much much cooler.
5) Kill Streaks: Obviously this is one of the coolest new features. You can select 3 out of the list to use at each kill straek mark. The max I've killed is 20 guys in a row without dying so far. I think hitting 25 and landing a Tactical Nuke will almost never happen.... You unlock new kill streaks simply by leveling. I think when you hit 70 you will have them all unlocked. (Yes there are 70 levels now)
6) Care Packages - These are a huge new dynamic of the game. Basically at 4 kills in a row (or 3 with Hard Line Perk) you call in a care package. This is a flare you drop on the ground... a few moments later a helicopter flys over and drops a package on the ground (watch out though - if it lands on your head you die!). This also requires you to be in a "safe zone". Don't drop it where an enemy is - or he can shoot you and steal it!! ( likewise you can steal enemies crates - I stole a shit ton of them too)
This package is either a random kill streak reward OR full ammo. So far I've gotten, UAV, Counter UAV, Predator MIssles, Chopper Gunner, AC130, Stealth Bomber strikes - EVERYTHING pretty much except tactical nuke... I HOPE you can get a nuke in these from time to time
** Special note here. Sentry guns ROCK!! You set these bad boys up and they MOW ****ers like buttah!
But anyway - when you drop one - people come out of the woodwork - it's a lot of fun.
7) Death Streaks - This one is more handy than I thought it would be. Even in games I'm doing well in - I find that I die a few times in a row quite often. A 10 second Juggernaught after death can come in handy. Or use a Martyrdom - or instantly come back to life when you die - (some require 4 deaths) - all pretty cool to offset dying over and over.
8) Better game joining. Hmm how else can I word that. Basically you set up your team....click find match - and ALL of your team goes into the "Finding Game" menu. It's no longer just the party leader. This seems to have helped a lot with games where everyone wasn't able to get in a the same time - we DID have a few problems but it seems much better now.
CONS:
Ok, so there are a few negative things to the game. Pretty much all of the above negates these but they are definitely worth mentioning.
1) No XBOX Live Party Chat while playing Team Deathmatch and several other game types.
The game actually makes you switch out of party chat and into the normal game chat in order to play. This is HORRIBLE - utterly stupid. We have countered this by subsequently muting every person in the game besides our teammates - or at least the other members of your team - and WALLAH! Party Chat is back!! Luckily this is easy to do and pretty fast...but still annoying. Thus the email I sent last night about chatting only with people on your friends list....
2) Spawning - It's GOOD in the game but not GREAT. Several times I was spawned in front of an enemy only to die instantly.... Hoping this is addressed later.
3) Did I mention there were only 5 custom class slots??
4) Leveling. Leveling seems slower than any other Call of Duty before this. This has me worried about wanting to Prestige. If we don't get anything from Prestige - I will not be doing it this time around. After playing for 30'some hours - I think I was level 41 or 42 out of 70 and I generally level 2x faster than the average joe. (yup im braggin now)
5) Didn't use a couple of great WaW features. As a party leader - I cannot leave a game and bring my party with me. I need to tell everyone to back out. WaW had a "Leave with Party" feature...why didn't they add this? :(
6) Party Privacy. Again - as a party leader, I cannot set my party privacy. Some random guy who played with us before joined our party - I didn't have an option to kick him...so - we played with him. He subsequently went AFK during the middle of a game to "make dinner". (i'll check the menu's to see if there is something I'm missing - I hope so).
7) Some people will say you "die too fast" in this game. Honestly I don't even know if there is much of a difference from previous CoD's but it might be worth mentioning that people seem to die "slightly" faster. Honestly I think they did away with a lot of the "shit guns" and opted for having all really good guns available - perhaps this makes it seem like people die to fast. Maybe this should go under PROS?
So that's about it. How many times did I say BAD ASS?
The game rocks bro. Get in the game as soon as you can - There are a lot of cool things that I'm sure i missed in this email...
Feel free to ask questions though
I'm leaving work in 40 mins to go home and play some more
Have a safe trip home Mr.
-EMBALMER