I had a bad string of phones last summer - started with my og Droid in March but I went through 7 of them - 5 refurbs, then a new Droid 2, then the Fascinate - until September. When my brand new Droid 2 crapped out I was absolutely fed up with Motorola. I had my choice of any phone in the store and I picked the Fascinate because they advertised it to be way better than it is. The screen's nice, the design is okay, but it's so light, plasticy, and chinsy. Since day one I've had something rattling inside my phone whenever I slightly move it. It's so outdated after almost 6 months it disgusts me. I guess I wanted to see if I was the only one completely underwhelmed by this phone, and if anyone has ditched it and paid retail for something better.
I know exactly how you feel about it seeming outdated pretty fast. I was sick of all the issues with the GPS and erratic behavior caused by all of Verizon's added junk. It was very frustrating to watch the phone get WORSE with new maintenance updates that would fix one thing but break something else in the process.
Once I rooted the phone and upgraded to the super clean Froyo leak, for the first time the phone actually started to feel like it was running the way Google intended it to - fast, intuitive and clean.
I keep mine in a Verizon shell that lets it lock into a belt holster. With that added it feels very sturdy. I was never a fan of the design of the back cover - like you I think it makes the phone feel flimsy. But with the shell attached it is very sturdy.
Your rattling might be the volume rocker, and not something inside. Hold the volume rocker with your thumb and shake the phone to see if it still rattles. If it does, something may indeed be loose inside, and the phone probably needs to go back to Verizon before it fails. If it's just the volume rocker, it's no biggie.
On this phone, rooting has reached the point where it is not too difficult and doesn't take a long time. With root access, you can back up your phone before you apply any major changes or updates, and always go back to what you have today if you experience problems. Once you have root access, you need to install ClockworkMod (CWM) Recovery to back up the phone's OS, apps, and configuration (also known as a "Nandroid Backup"). Titanium Backup is an application that is used to just backup apps and settings. With these backups you can go up/down/left/right with OS upgrades, downgrades and enhancements and always get back to where you were before you started. I now do a Nandroid backup at least a few times a week, and before I do anything to the phone that may have the potential to not work out, so I can go back to a working setup very quickly.
One of the cool things about having a rooted phone and installing new OS versions and enhancements for me is that every time I load an OS or enhancement it feels like I'm using a new phone. It gets addicting if I'm not careful - eventually I have to stop and say "IT'S GOOD ENOUGH NOW!" and just enjoy the phone.