Not to throw anything away from your post, but I've got a Nexus One and don't have any of the issues mentioned. I generally don't reboot the phone (sometimes it'll be up for weeks as long as I keep to a proper charging schedule). Stock Android has everything i need, so much so that i consider the fact that it has stock Android to be a _feature_ (and anything else, i.e. Motoblur to detract from it). I'd be pointing to Motorola for your issues; there are phones out with rougly the same specs that run really smoothly (i.e. HTC Desire Z - 800Mhz processor as well but is snappy).
My friend does have a defy and i've noticed the lag. Whether it's just Motoblur (which is still garbage and just looks ugly, why do they keep including it!), or HTC are doing some optimizations that Motorola isn't, who knows - I still won't be buying a Motorola phone because of their poor upgrade support!