I've looked over a list of new ICS features, and so far I'm not impressed. Most of the things are pretty pointless, like Face Unlock and camera right from lock screen (it disables PIN so why would you ever use it). Others are pretty minor, like notification swipe and quick respnse. The only things significant seem to be data management and folders. Other things are even harmful, like only five homescreens. I might be missing some things so please remind me.
All in all, ICS is nice and all, but it's not spectacular enough to skyrocket the Galaxy Nexus into "best phone in 2011." Especially when the Galaxy S2 has better specs in almost everything and will be getting ICS as well.
It's not really any single feature, it's a combination of many little things that together make the phone really, really pleasant to use. Everything is fast, smooth(yes they are two different things), intuitive, and it looks nice.
I also find it odd when people say things like "Galaxy S2 has better specs in almost everything". Let's break it down shall we?
The OMAP 4 is the same processor as the Exynos; they are both unmodified Cortex A9 CPUs with NEON extension support. A lot of people pass along the myth that the Exynos is some magical super processor, it is not, it is just a Cortex A9 coupled with a Mali 400 GPU. The only processing advantage an S2 has is the Mali 400 GPU which is in fact better than what is in the Galaxy Nexus but not by a whole lot. Many Galaxy S2 phones(the highest end ones) use a Snapdragon chipset which has both an inferior CPU and an inferior GPU to the Galaxy Nexus.
The Galaxy Nexus screen completely blows away the screen on the Galaxy S2, no debate here. Higher resolution, higher PPI, larger.
The S2 has a rear camera that takes a bit better photos than the Nexus but the Nexus still takes quite nice photos for a phone. It can take pictures significantly faster than the S2 and it can also take pictures while recording video at the same time. Personal preference on this one; I could see different people caring more about either one of those benefits more than the other.
The S2 uses Touchwiz, not vanilla Android. ICS on the S2 will also be using Touchwiz. Leaked videos of ICS on an S2 show that it looks almost identical to what is on the S2 already. Nonetheless, this is personal preference and can be changed if the user is willing to put some work in. Besides this, the S2 is doomed to be months behind the official Android releases just like every other non Nexus phone.
Galaxy Nexus has Near Field Communication support, the S2 does not.
The GN's 32GB of onboard storage is unified. Meaning all of that can be used for anything: apps, media, whatever. There is no artificially set small partition of a couple GB for apps.
My sister got an S2 yesterday actually. Is it a nice phone? Absolutely, it's incredible. Would I choose one over the GN? Not a chance. Would I use one if the GN didn't exist? No doubt. My $0.02.