I spent a few hours over the weekend doing challenges and stuff with the keyboard, and I think I've got the hang of it. It will probably take more practice before I can use the keyboard effectively in continuous battle, but I was surprised how quickly I took to it. It's probably because I was already good at vim...
Basic movement feels a bit harder, but I think that's down to familiarity. Switching between standing and crouching block is a bit weird, because holding down the keys just feels different from using the stick. And a few special moves are harder, such as the hcb, f moves: I have to complete the hcb, release the keys entirely, then hit f+attack. But when I look at the results in a keyboard input display, such as https://www.microsoft.com/appliedsciences/KeyboardGhostingDemo.mspx, I can see my error: the hcb is "fds", and if I don't completely release the S key before hitting F again, I'm holding two opposite directions at the same time, and the game system doesn't recognize the forward direction.
On the other hand, some kinds of directional inputs are easier. With a stick, sometimes moves are harder to do when facing a certain way—with the keyboard, that concern largely disappears. Things like instant air dashes become a lot easier. And combos which rely on combinations of directional and attack inputs become much easier. Basically anything with a mix of crouching and standing attacks feels easier, because having two sets of buttons makes it easier to sync up your hands to your brain.
Like, with an arcade stick, your left hand is doing one thing (moving a stick around) and your right hand is mashing buttons. With the keyboard, both hands are mashing, so doing complex combos means you're at least doing the same basic motion with each hand. It just feels easier. Also, with the keyboard, you can jump either thumb—so for movement, you tend to jump with your left (movement) hand, but when doing a jump-cancel combo, you can hit jump with your right (attack) hand, and get the entire combo motion coordinated between just those fingers.
I am probably overanalyzing this. But one final note: certain SF4 combos were just too hard for me on stick. Like, Dhalsim's non-stretchy mk xx Yoga Flame requires you to hit b+mk, then instantly slam the stick forward and do a half-circle back again. With a keyboard, I think I would land it every time.