[Nexus 5 Q&A, Help & Troubleshooting] someone0:
I got a chance to re-read your post again(with addition info) along with the comments from other. Since this post is a prep for rooting, I'll give my view in regard to that.

I saw you answered about the wiping the dalvik cache, which is right, but maybe explain what a dalvik is, and why sometime we should wipe it. I guess you can consider this as a basic trouble shooting step if a person have a custom recovery and the Android OS acting up. I think you know what a dalvik is, but there is a wiki article also.

To summarize, Android is an open source OS, which support many different architecture. While it's mainly use in the ARM architecture, it could run on x86 also and even with in the ARM there are still some variation. Just like in x86, there are many extensions to the x86 and SSE is an example of that; ARM is no different and there are some variation from one ARM CPU to another. Dalvik is basically a VM(Virtual Machine) engine, this keep the apps to be universal across many architecture. While this is all an good, but there is a cost to this, processing power cost that is. Meaning, if the apps have to run through VM everytime, it will be very slow. Dalvik cache is basically cache of those apps that's already gone through the VM thus it doesn't require to go through the VM everytime. And when an app run, it run from this version instead. Since each ROM may contain different version of app or optimization or even different kernel or Android version, using dalvik cache from different ROM can get you into trouble. Think of it like trying to run application designed for Windows XP on Windows 7 or Windows 8. It may works, it may not works, or it may even crash. Wiping the Dalvik cache will force the Android OS to optimize all the install apps all over again.

And here is a tip. Never try to make any system modification(root or flash ROM/recovery) without preparation for time and resource to troubleshoot. What does this mean? While most of the time, this is relatively painless when the you follow the instruction carefully. That said, things can go wrong. And sometime, things can go very wrong. You may accidentally drop your device and interrupt the process or you flash the new ROM and it's not compatible with the app you need. So, don't just say I saw a new ROM when I woke up and I wanna try, and you have 30mins before going to work. Even if it only take 5 minutes to flash the ROM, if thing goes wrong that 25mins you have left may not be enough to fix your problem and you have no phone to use that day. And your business partners or your boss can't contact you. That maybe a good things for some people, but I don't think that's the case for most people. You may not want to try to root or flash ROM/recovery when your phone is the only device that can access the internet at the time. If something goes wrong, you want to be able to get online and find a solution. I preferably to only do this kind of things when I have access to the PC with the necessary drivers installed. If I need adb or whatever tool, it will be there to back me up. Just because you can download the ROM straight from your phone and flash it w/o the need for PC that doesn't mean it's a good idea to not have a backup in mind.