For those of us who don't necessarily want to play high fantasy and/or sword & sorcery games all the time, D&D's general lack of black powder weapons can be frustrating and jarring. Many of the weapons which AD&D adventurers can use (the guisarme, the lucerne hammer, the bardiche) came into being at around the same time that black powder arms were becoming more common on the battlefields of Europe; it seems odd that esoteric polearms should be given such preference over the humble hand cannon. Admittedly a hand cannon isn't the perfect weapon when you're doing what D&D adventurers usually get up to, but it seems a heck of a lot more useful in those circumstances than field plate, an awl-pike, or a fauchard. I suppose the D&D creators (and most players) just don't or didn't think that guns 'fit' with the tone they were aiming for. (I know that 2nd edition lists arquebuses in its equipment pages, but that's all there is.)
The Warhammer creators had no such compunctions, and Warhammer fantasy battle is full of weird and wonderful black powder devices, from ordinary arquebuses to cannons, rocket launchers, and flame-throwers. I like this. Although Warhammer is a fantasy setting you get the feeling that technology develops in at least some sort of haphazard way - unlike in Greyhawk or the Forgotten Realms, where it seemed to reach a certain level and just stopped. For thousands of years.
Of course, this feeds into the old classicist/romanticist, or banalifying systematiser/mysterious dreamer debate. It's perfectly fine for there not to be cannons in The Lord of the Rings or the Corum stories, because those aren't meant to be in any way an accurate reflection of reality. But if you like a heavier dose of realism in your fantasy pie, their absence smacks of grandfathering rather than anything else.
One of the reasons I love D&D is that I think it can be all things to all people. Some people will tell you that games should be narrowly focused towards certain 'goals', but I've never subscribed to that view. What makes a game good is a good DM and good players doing what they want to do with whatever system they've chosen (though some systems are undoubtedly worse than others). The D&D designers always seemed to have this philosophy, at least up to and including 3rd edition. So to me it seems a little odd for the core rules to implicitly dictate that black powder weapons don't really fit with the D&D vision of what fantasy should be. I sometimes want to play a bloke with a massive matchlock gun which takes ages to load but which can knock an ogre's head off at twenty paces. It would be nice if D&D allowed me to do that without having to come up with my own rules.
The corollary of this is: what kind of guns, cannons and other war machines would kobolds, mind flayers, derro or the other creatures of D&D come up with? The various races of Warhammer have eccentric devices which suit their 'national' character: Chaos Dwarfs have huge cannons which cause the very earth to quake; the empire has steam-driven tank-like contraptions with mini-cannons; the armies of Chaos have fire-belching machines inhabited by the souls of the damned, who are always likely to be driven insane and start attacking their comrades. Once you start thinking about the introduction of firearms into D&D a whole new world opens up. Githzerai arqubusiers, thri-kreen pistoliers, ogres carting around battered hand cannons - the possibilities are fascinating.