Oh, for the love of all that is good, why do I do this to myself?! Lol At this point I'm not even sure I should get the phone... I mean I've never even SEEN a nexus. How do I know I will like it? But all this hype makes me want it in the worst way. Google sure knows what it's doing.
Honestly you can look at previous Nexus devices (Nexus One, Nexus S, Galaxy Nexus, soon to be Nexus 5. See how they and Android evolved) up to the most recent one, then look at your current device and other top tier phones like the S4 and HTC One. You can compare the pure Android experience vs the skinned version on Samsung and HTC devices. When it comes to performance, smoothness, visuals of the OS, Nexus wins every time.
Think of it as the origin of Android. As if there was no other manufacturer making their version. Like Apple is the only company behind the iPhone. You don't see Samsung, LG, HTC, etc making their custom version of iPhone's. So you can expect the latest updates and great performance from your device straight from Google until that device is no longer supported, because software eventually beats hardware. For example the Nexus S is no longer supported as far as updates because it's a pretty old phone now and couldn't handle it. The fact that Android is an open system they allow others to customize their own version with manufacturers. Apple is closed source.
In comparison to features though, Nexus devices lack the gimmicks. The camera itself was never that polished as far as quality and lacked mainstream like features in comparison to Samsung's hand gestures, supper packed camera features, HTC's camera features, Boom Sound, etc. Pretty much some of the gimmicks or even useful features that are the selling point. This time around it looks like the camera will be of better quality judging from the leaked photos taken with the Nexus 5. Personally, leaked photos taken with the most anticipated handsets never are the best. You have to see in a real review or test on your own once purchased.
Other then that it will have the latest specs matching the high end phones of today. Large screen (1080), clean design, quality camera, decent size battery (OS optimized for good battery life assuming), etc.
I hope my novel of a response helped somewhat with little to no confusion
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This image is the basic version
https://twitter.com/phandroid/status/395559204133404672/photo/1