Ran into a few "quirks" on my N900. I'll call them that, for the term "shortcomings" is a little bit harsh, on the grounds that others obviously had/are having success with these and damning the N900 because I'm personally having difficulty is short-sighted.

-Lack of MMS (or at least for me)
This isn't really that big a deal for me, but it really surprised me initially. A friend wanted to text me a picture, and I never got it. A wiki page+blog post+AT&T MMS site (tried both Android and Quick Messaging setups) later, I have fMMS on my phone with a setup...and I still can't get MMS. IDK what I'm missing, but it's not working.

For functionality's sake, I'd feel more comfortable having the ability to read MMS.

-The Lockscreen (or lack thereof)
Something that also surprised me was the initial lack of a lockscreen when the screen's turned off. I'd rather have one. So, I installed QtLockscreen, set a password, and...no dice. I'll get prompted for a password, but no lockscreen. I'm kind of stumped here.

-BlueMaemo
This is the best app I'll never use. This, combined with Pierogi and transfering files over WiFi+BT would be killer. Having a combination of features on a 5-year-old device that either don't exist today or others pay a premium for? Amazing. However, it requires EFL libraries that either aren't in the repos or FAM/apt-get refuse to install. I saw a forum post that had the libraries available for download, but I just don't feel like going through all that.

-Battery Life
After uninstalling Catorize, I missed the hell out of it, so I put it back on, measured battery usage, took it back off, measured battery usage again, and found no significant difference in batttery use. If there is a difference, it's not convincing me that I should uninstall it.

I did notice, however, that the N900 is usually operating at peak performance when off the charger. I OC'd it to 805MHz (I feel it's a sweet spot regarding stability and performance), so perhaps that's an issue. Perhaps it's just that the official Nokia batteries are complete crap, and I should immediately get another battery. Perhaps that dual scud setup people are using, but the phone's already on the thick side. Perhaps that PolarCell battery could come in handy.

-Charging the phone
After switching chargers, every problem I've had with the phone to that point (not charging while talking, not charging while OC'd, cannot charge if the battery's flat, the fact that the uUSB connector bites down hard) pretty much vanished.

I use an HTC USB wall charger (very small, to boot) with a uUSB cord that has smaller prongs than the one on the Nokia charger. That solved everything. I know people have said to use the official Nokia stuff, but doing the exact opposite has gotten me positive results thus far.

-Notifications
So what works? SMS, MMS (even though I can't view them), and IM do, but it's strange. MMS alerts go off long after I've recieved the MMS. IM will do this occasionally, but the frequency of IM doing it seems to be going down over time. SMS is pretty much perfect, but it'll keep going off when I get a new message.

Email, however, is a different story. I set my Gmail up with the native email client, but I get no notifications. I'm not sure if that's the default behavior or not, but it's bothering me. Having to check my email all the time is a pain, so I'd much rather it do so automatically.