Well, you will not have to worry about bricking this phone at all. Unless when you say you bricked your Droid 2, you mean that you had to factory reset it to get it work again, in which case, that is something you have to get used to when flashing ROMs. This is because, to answer your other question, the proper way to flash ROMs is manually, in Clockwork or whatever custom recovery you choose, after performing a factory reset and cache wipe. Using apps like Rom Manager can cause errors or bugs in the flashing process, it's best to download any ROMs yourself directly to your SD card and flashing it manually via recovery. The Nexus is a developer phone and the unlockable bootloader leaves it pretty much unbrickable, because you can just reflash stock images at any point.
As far as batter life tweaking, this comes usually in the kernel, which there are TONS you can try out from many developers. I personally switch back and forth between Franco and Imoysen, but I stick with Franco because he has more color tweaking options and my Nexus suffers some unfortunate amounts of yellow-tinge banding without all the color tweaks provided by Franco's kernel. But it does also bring great battery life performance. ROMs themselves can also bring better battery life, you just have to experiment, as everyone's phone and phone usage are extremely different.
ROMs, the answer is Jelly Bean ROMs. They're awesome, Jelly Bean is awesome. I am currently running Liquid Smooth Jelly Bean Beta2. It's a solid ROM. There's still some bugs, but they are not major and do not effect the normal functionality of the phone at all. This is another area you have to do a lot of research, as ROMs won't function the same for all phones as well. But this is the fun of it all! Because all these different ROMs out there bring a plethora of different features, you just have to to find out what you like. Just research the threads of any ROM you might be interested in, they will have tons of info.