Samsung Galaxy S2 Vs. Galaxy Nexus

Galaxy S2 has a Faster CPU
Dual Core Exynos 1.2ghz is A LOT faster than Dual Core 1.2ghz TI OMAP 4460

Galaxy S2 has a Faster GPU
Not only the Galaxy Nexus has a slower/older GPU, but it is a slower GPU on a device with a lot more pixels. Bad combo. Expect noticeable difference on any moderate GPU intensive app. People are saying how the Galaxy Nexus runs graphic tests slowly because it is not optimized. This is true. Keep in mind that pretty much the Galaxy Nexus is the only phone with 1280 x 720 resolution right now, and the apps, to get the full potential of the screen will have to be optimized. Realistically all apps will not be optimized at first for the Galaxy Nexus. And slowly they might become optimized. And some may never be optimized because it is up to the developer. Also, after optimization, remember it has more pixels to process with slower hardware than the Galaxy S2.

Galaxy S2 has a better rear Camera
8MP > 5MP
Sure it is not all about megapixels, but did anyone say 8MP on the S2 was bad? Not at all, it is fantastic. 5MP sensor on the Galaxy Nexus might be a great sensor but it is a downgrade in image quality from the 8MP galaxy S2 sensor. I'm sure the Galaxy Nexus 5MP sensor is better than a lot of other 8MP sensors out there though.

Galaxy S2 has a better front facing camera
That is 2MP on the S2 and only 1.3MP on the Galaxy Nexus. Again, it is not all about megapixels, but speaking from first hand long term experience of having a older Samsung Galaxy S with a 1.3MP front facing cameras, the 1.3MP camera has been one of the most unimpressive cameras I've ever encountered. Galaxy Nexus is most likely using the same 1.3MP camera. I can say that the 2MP Galaxy S2 front facing camera is a lot better than the older Galaxy S front facing camera by comparing the quality of the image using the mirror feature on the OS.

Galaxy S2 has Super AMOLED Plus (Better than Super AMOLED (Pentile) on the Galaxy Nexus)
Might be less important because of pixel density, but take it from a user who has used a Galaxy U (a Galaxy S with a higher pixel density because of the smaller 3.7" screen...~252 PPI) and used it for a whole year. You would think with a higher pixel density like that you can't see the checkerboard pattern? Wrong, it is visible. Clearly visible when white lines are visible on the black background. Are there white lines common on black background on android devices? Duh! But you'll get used to it like you get used to scratches on your glasses. Maybe ICS will mask the ugliness with the blue glow.

Galaxy S2 has microSDHC (comfirmed to be compatible with microSDXC) slot while Galaxy Nexus has none.
So right now, you can put an additional 64GB card on the Galaxy S2 on top of the 16GB. The Galaxy Nexus will be locked to 16 or 32GB. Galaxy S2 might one day support up to 2TB, but more realistically, people will be carrying 128GB and 256GB cards during the lifecycle of the Galaxy S2 device.

Galaxy S2 has been announced to get Ice Cream Sandwich Android OS 4.0 update.
Sure if you get the Galaxy Nexus, you can have ICS for 6 months maybe. But is it worth the sacrifice of hardware? Software you can upgrade. Hardware cannot be changed.

Galaxy S2 has native DviX/XviD/MKV support. On Galaxy Nexus you will need to do software decoding which is much slower. A lot of times the HD videos are too slow for watching videos.
The Galaxy Nexus will be able to play 1080p h264 files, but it will certainly not be able to play any 1080p mkv or divxes like the Galaxy S2.

Galaxy Nexus has a bigger screen. I hear it is awesome by all accounts.

Galaxy Nexus has a much higher resolution
I bet people are drooling with the thoughts of watching 720p videos natively on the device. But they are forgetting that it has to be encoded in h264. So basically you have to shoot the video yourself, or encode it yourself because 720p h264 is not as popular of a format than 720p and 1080p mkv files that Galaxy Nexus doesn't support. This will be a serious letdown for many thinking they can just download any HD content and watch it on their device.

Galaxy Nexus has a lag free camera
A lot of people are speculating ICS 4.0 will bring a lag free/near lag free camera to a lot of devices. So this may be only a short lived advantage, just like ICS being a reason to buy a Galaxy Nexus.

In conclusion, the Galaxy S2 is superior almost every way possible. Where Galaxy Nexus is better, it seems to be crippled by Samsung choosing to go with inferior screen technology (SAMOLED instead of SAMOLED+), a weaker GPU, and the decision to only support a limited set of HD codecs (no mkv support). As for Samsung Galaxy Nexus offering features that are only possible because of it having Ice Cream Sandwich (Android OS 4.0), the S2 has been cofirmed to get this upgrade in the future. Again, smartphone software can be upgraded, smartphone hardware cannot.