I have all sorts of weirdo bugs, and I am on an original iPhone (4gb)
1) Safari crashes: usually after more than 15 minutes of prolonged use. Not often that I use it that much, so it's a minor point.
2) Lag in the UI. If I haven't rebooted recently, this happens every time I switch apps or return to the home screen. Very annoying.
3) Phone app/SMS/Mail lags: Argh. Wildly unpredictable, except that if I need to call someone or answer an important text or email, its pretty much a lock that there's a delay.
4) Twice now reboots have ended with a "connect to itunes for activation" message. The first time I freaked, turned it off and then back on, and it was fine (and I got a pop up that said "iPhone is activated.") It happened again the other night but I wasn't surprised by it this time and the same thing happened (reboot got me back to normal). What worries me is why this happens...
5) App store "quirks". Never knowing if it's going to let me download an app or just leave me in an endless loop of buy/install/buy/install/buy/install where the stupid install button turns back into buy, and then eventually it just tells me the store is unavailable. Buh?
So even v1 iPhones are not immune to the strangeness.