Let's look at the history of the series, and what changes they made. It will be clear from my comments where I think the series started going wrong. (Spoiler: it wasn't Most Wanted).
The Need For Speed:
It was a simulation/Arcade Racing game but it had realistic Engine sounds and handling, at least more so than other games of the time, which focussed on arcade gameplay. Additionally, many of the cars available were in fact common street cars (with the exception of the Lamborghini probably). It was a good game that for it's era was a very good simulation racer. It was hard as heck though.
Need For Speed II:
This Game had several modes of play, and also appealed more to the Arcade style. It had a "simulation" style but the actual game mode seemed to affect handling and braking more than anything else. Another major change was that it went with high-end exotics rather than more "attainable" cars. I like this one as well. It's exotic cars may be a departure from it's previous installment, but it certainly held true to it's title.
Need for Speed III:
Expanding off of II. In fact, NFS III was inspired by a cheat from NFS 2 that caused you opponents to chase you around. NFS3 as it's subtitle, "Hot Pursuit" indicates, is the first game to feature the presence of Police cruisers and vehicles on courses, to chase about and ticket offending drivers for going over the speed limit. As expected the game is a lot of fun to play and has that understandable amount of ridiculousness that you can almost see occuring in real-life, such as committing vast resources to catching speeders in a fashion that probably endangers the public more than just letting them race; though the speech track and "cop scanner" were a nice touch. This game was also a bit heavy on the specular environment mapping. One secret level and a secret car acquired in much the same way as in the previous game round it out well. Even so it still feels a bit rushed and low on content.
Need for Speed: High Stakes (or Road Challenge, depending on where you live) on the PC at least is pretty much NFS 3 reimagined and 'finished'. It doubles the number of tracks, adds new cars, a career mode, vehicle damage that affects car performance, locale-specific police speech tracks round out that portion of the game. It also has all the levels from NFS3 included, though they are available as unlockables. Personally this is probably my favourite of the series. Only downside is that while it's an improvement over NFS3 it is still from ~1998 so in terms of today it's graphics are out-dated, and also the game gives you a hard time on newer systems sometimes.
Need For Speed: Porsche Unleashed
This was an interesting spin-off game that focussed heavily on a single brand of Car, the Porsche, which is hopefully clear from the title. What I found most interesting about this game was the extreme attention to detail; hazard lights, blinkers, realistic handling, event doppler-effect sound. The tracks also were from real-world settings, too. A very good game, though it has even more trouble than NFS4 on modern systems.
Need for Speed Underground
After Porsche unleashed, EA took a good long look at their franchise, and they realized that it didn't appeal enough to the wannabe gang-bangers and Rich white kids that liked to pretend to be hip. So they took the NFS franchise and did a point by point look at it, and for each part of the design, they asked themselves, "What would a douche do to their car, and what would they do with it" and added that into the game.
If one cannot tell, I think this game and the direction it went are absolutely horrible. I thought this game was pretty crappy in comparison to it's predecessors simply because it was so clearly immersed in the culture of rich white inner-city kids with too much money and time on their hands. This pretty much extends until Shift.
As far as I'm concerned, Need For Speed Underground 1 &2, Most Wanted and Carbon are trashy games for a trashy audience. In fairness they play OK for what they are, but IMO they aren't really "NFS" games in the traditional sense.
Shift and Shift 2 seemed to take the right direction to fix this. By actually appeal to real car enthusiasts (somewhat).
That said, there is something to be said of exploiting the niche that is players that want to immerse themselves in rich white-trash hipster douche culture.
My name is Ozymandias, king of kings. Look upon my works, ye mighty, and despair.
The Need For Speed:
It was a simulation/Arcade Racing game but it had realistic Engine sounds and handling, at least more so than other games of the time, which focussed on arcade gameplay. Additionally, many of the cars available were in fact common street cars (with the exception of the Lamborghini probably). It was a good game that for it's era was a very good simulation racer. It was hard as heck though.
Need For Speed II:
This Game had several modes of play, and also appealed more to the Arcade style. It had a "simulation" style but the actual game mode seemed to affect handling and braking more than anything else. Another major change was that it went with high-end exotics rather than more "attainable" cars. I like this one as well. It's exotic cars may be a departure from it's previous installment, but it certainly held true to it's title.
Need for Speed III:
Expanding off of II. In fact, NFS III was inspired by a cheat from NFS 2 that caused you opponents to chase you around. NFS3 as it's subtitle, "Hot Pursuit" indicates, is the first game to feature the presence of Police cruisers and vehicles on courses, to chase about and ticket offending drivers for going over the speed limit. As expected the game is a lot of fun to play and has that understandable amount of ridiculousness that you can almost see occuring in real-life, such as committing vast resources to catching speeders in a fashion that probably endangers the public more than just letting them race; though the speech track and "cop scanner" were a nice touch. This game was also a bit heavy on the specular environment mapping. One secret level and a secret car acquired in much the same way as in the previous game round it out well. Even so it still feels a bit rushed and low on content.
Need for Speed: High Stakes (or Road Challenge, depending on where you live) on the PC at least is pretty much NFS 3 reimagined and 'finished'. It doubles the number of tracks, adds new cars, a career mode, vehicle damage that affects car performance, locale-specific police speech tracks round out that portion of the game. It also has all the levels from NFS3 included, though they are available as unlockables. Personally this is probably my favourite of the series. Only downside is that while it's an improvement over NFS3 it is still from ~1998 so in terms of today it's graphics are out-dated, and also the game gives you a hard time on newer systems sometimes.
Need For Speed: Porsche Unleashed
This was an interesting spin-off game that focussed heavily on a single brand of Car, the Porsche, which is hopefully clear from the title. What I found most interesting about this game was the extreme attention to detail; hazard lights, blinkers, realistic handling, event doppler-effect sound. The tracks also were from real-world settings, too. A very good game, though it has even more trouble than NFS4 on modern systems.
Need for Speed Underground
After Porsche unleashed, EA took a good long look at their franchise, and they realized that it didn't appeal enough to the wannabe gang-bangers and Rich white kids that liked to pretend to be hip. So they took the NFS franchise and did a point by point look at it, and for each part of the design, they asked themselves, "What would a douche do to their car, and what would they do with it" and added that into the game.
If one cannot tell, I think this game and the direction it went are absolutely horrible. I thought this game was pretty crappy in comparison to it's predecessors simply because it was so clearly immersed in the culture of rich white inner-city kids with too much money and time on their hands. This pretty much extends until Shift.
As far as I'm concerned, Need For Speed Underground 1 &2, Most Wanted and Carbon are trashy games for a trashy audience. In fairness they play OK for what they are, but IMO they aren't really "NFS" games in the traditional sense.
Shift and Shift 2 seemed to take the right direction to fix this. By actually appeal to real car enthusiasts (somewhat).
That said, there is something to be said of exploiting the niche that is players that want to immerse themselves in rich white-trash hipster douche culture.