DiRT 2 approaches everything in a similar way and is overall a better, more fun game to me. There are some design changes that are good or bad depending on your personal tastes but you can't argue with the core of the game which is it racing gameplay that comfortably straddles the fence between hardcore racing simulations and arcade boosters.
The first thing you'll notice about DiRT 2 is that its presentation style is very different from the original. The X Games play a prominent role this time around, and the spirit and attitude of that competition shows through here. Instead of just picking races from a menu, you actually 'travel the world' in an RV that serves as a hub for different race types, on- or off-line play, areas showing your stats, and other relevant information. After choosing an event to compete in, you'll actually step outside of your camper and into a crowded little lot among a bunch of other racers, a crowd moving by past the barriers, and a ton of advertisements, half of which seem to be for Monster energy drink. I feel like they did a good job "modernizing" the game's UI style and incorporating 'extreme sports' elements without going over the top, but I have already read some negative reactions from people that can't stand the words "dude" or "bro".
Graphics-wise, DiRT 2 is a great looking racing game. Every track looks good, but the ones with a lot of vegetation or architecture really stand out. This game seriously has some of the best looking virtual foliage I've ever seen. The licensed cars and trucks look very nice, with lighting playing evenly across them and damage & dirt modeled well. While the game has dynamic lighting, lens flare and other expected effects, none of them are overused or distracting. Dust clouds behave appropriately, and the little water pools in the pits of some tracks look awesome. If you're driving from the 'helmet cam' view, water and mud will sometimes actually spray onto your windshield, obscuring vision for a moment before the wipers kick in. The game's frame rate is very consistent. Even with 8 vehicles on the screen there was no noticeable slowdown. The HUD contains a lot of information and can be distracting at times, but every element can be turned on or off.
DiRT 2's sound is overall fine. The music covers a small range of different types of rock music, and while it never stood out to me it also never became annoying. A few of the artists I can name off the top of my head are Queens of the Stone Age, Green Day, and Eagles of Death Metal. The sounds of the racing are good--you can tell exactly what type of surface you're driving on by the way it sounds. Most of the engines are nice and gravelly, and the tire screeches and squeals sound real without being intrusive or overbearing. Actually, I take it back--the Scars Over Broadway song is kind of annoying after awhile
Game play is the most important part of any racer, and I really feel like DiRT 2 does an even better job of combining sim- and arcade-like aspects in to a unique package that can be enjoyed by fans of either or neither. The game is more forgiving this time around, allowing 'Flashbacks' that you can use from the instant replay screen. The number of flashbacks you get is determined by the difficulty, with Easy getting 5 and the highest difficulty getting none. The driving itself is also fairly forgiving as it seems the developers knew many people would be playing with a gamepad's analog sticks and not a driving wheel. Understeer and oversteer aren't quite as problematic here as they were in the original DiRT and other games like Forza. With some minor tweaking, you can run really good racing lines with a regular controller. There are 6 difficulties to choose from for any race/event, and I feel like they scale really well. The AI in this game is also a little more aggressive here, with rival drivers being unafraid to bump and jostle you the way you do them. On the lower half of the difficulties, they will even pull off some bone-head maneuvers that will have them careening in to walls, flying off jumps sideways and rolling past you on landings.
The available racing disciplines in DiRT 2 are more varied than the original game's, but at the same time there are less of them. I especially miss the hill climb events from the first game that had the ridiculously over-powered cars. No more semi racing either. What you do get are Rally, Rally Cross, Raid, Landrush, Trailblazer (staggered point-to-point), Domination (where you race to get the best time in different sections of a track and win by holding them), Last Man Standing (there is a timer and after each countdown the last place competitor is dropped), and Gate Crashers (where race but also try to knock down the most gates on a course).
Career progression comes in the form of XP which you get by competing in events and fulfilling 'missions' like sliding a certain amount of feet or overtaking a certain number of competitors in a race. As you gain XP you level up which opens new events, and while you start on the Rookie tier for each location, you will eventually open up Pro and then All-star level events. No matter what difficulty you play on, the cars are faster and the AI generally better on each successive tier. A Rookie event on Easy may be a cakewalk but a similar event on All-star will require more skill even if you still play on Easy.
Depending on your progress and placing in events, different 'superstars' like Travis Pastrana, Dave Mirra, and Ken Block will sometimes challenge you to Throwdowns which are 1-on-1, 1-lap races, or they might task you with beating their 3-lap time on a certain course. These specific challenges don't pay money but do give good XP and increase your reputation with your fellow drivers. As you gain reputation by doing well in events, higher tiers allow team races where you can choose your other team member and your points for events are combined.
The multiplayer offerings in DiRT 2 are far more plentiful and engaging than in the original. You can race in real-time against other people in any discipline, earning Fame Points as you do (the online equivalent of Tour XP), as well as putting together your own party and creating a "jam session" where you can tweak the rules, set options and just generally screw around.
The game also has some nice little touches. Besides unlocking new vehicles and liveries as you progress, you can also earn trinkets like hanging your Avatar or fuzzy dice from your mirror, or a skull or hula girl to put on your dashboard. You can choose between 2 different co-driver voices for the rally races, and you can select whether you want them to call out in technical terms like the original game (right 1 into crest 40 to left 3 long tightens) or a simpler style (right hairpin to crest short to easy left, don't cut).
As good as the game is, I do have some complaints. First, there are less tracks than the original game, so you'll feel far sooner that you're doing the same runs over and over. Second, since there are less event types, there are also less vehicles. Third, even though it's less of an issue now, there are less options to tweak on your vehicle. No longer can you set the camber & toe of tires or specify a clutch type. Instead, there are simple 5-point sliders for Gear Ratio, Downforce, Suspension, Ride Height, Differential and Brake Bias. The game offers brief audio explanations of what each of these affect, but isn't half as thorough as the first game was. Lastly, even though the game allows for full damage, there is no real incentive to turn it on because it doesn't offer any reward for doing so. The amount of money earned for each event does increase with difficulty, but not by much. It's far more worthwhile to win a multi-race event on Casual or Easy than it is to place third on Hardcore or Savage, unless you're a masochist.
DiRT 2 has a good array of Achievements. The first 300Gs in the game are fairly easy, and the rest are just a matter of doing all the events. There are a few multiplayer Achievements but they aren't too difficult. It will take a few weeks for you to compete in 4 online tournaments, though. And winning 5 online Pro Tour races is almost as much about luck as skill.
Bottom line: DiRT 2 looks better, plays better and overall has more stuff to do than the first game, even if there are less tracks overall. The single player tour is more varied and fun, and the multiplayer is more robust.
If you enjoyed the first game, you'll love the sequel. Even if you never played the original DiRT, you should give 2 a try if you like racing games of any kind.