When I pointed out that the D5100 had "better" video than the D7000 I was mostly alluding to video format decisions: the D5100 can do 1080 24/25/30P while the D7000 can only do 1080 24P. On the negative side, the D5100 doesn't allow manual control of video, meaning you have to jump through some hoops to get video that doesn't constantly change ISO to deal with exposure changes. It should be obvious from my comments that I regard format choice more important than manual control.
Why? Because I can figure out ways to get around some (but not all) of the D5100's lack of manual control, while there's not a lot I can do about getting a different frame rate out of the D7000.
Let's talk about frame rates. I'm of the opinion that if a camera does HD video it needs to do at least three and preferably five frame rates: 24P, 25P, 30P are required, plus 50 and 60 are nice to have, preferably in Progressive (P) rather than Interlaced (i) format. If a camera doesn't have at least the first three formats (preferably in both 1080 and 720 frame sizes), then it isn't a fully functional video camera.
So what are those frame rates for? 25 is easy to explain: that's the PAL/SECAM frame rate. 30 is the frame rate of NTSC. Those represent the broadcast standards around the world. If your video is ever going to be transmitted over a broadcast transmitter or cable network, it's going to be at 25 (PAL/SECAM countries) or 30 fps (NTSC countries).
50 and 60 fps come into play in two ways: (1) if you're using progressive scan, you get a straight 2x slow motion capability (overcranking); and (2) if you're using interlaced scan you get 25 and 30 fps progressive or 2x slow motion in interlaced scan (this last is not typically used, as it doesn't hold up well). But we can do without these.
24 fps comes from Hollywood. It's the frame rate that motion picture film cameras typically use. You can stream 24 fps, display it directly on a computer, or convert it to film, but if you want to broadcast it in an NTSC country you have to resort to tricks that make it look a bit jerky: the dreaded 3:2 pulldown. Imagine that you're running at 120 fps (60i is essentially 120 "half frames" a second). You take four frames from the original and repeat one of them (technically half of one interlaced frame, half from another interlaced frame) to get 5 (5*24=120). The result is that three of the 120 fps "frames" will be your original material and two will have overlapping material from two adjacent original frames. Those of us who are trained in the motion arts can immediately distinguish 24 fps source material from 30 fps (here in the US) when broadcast. You can see it most easily in slow pans or motion, but once you see it it's annoying.
So why did we want 24 fps in our video cameras in the first place? Well, it turns out that audiences have about a century of training to recognize 24 fps as "film" and about a half century of visual training to recognize 30 fps (actually 60i) as "video." They do indeed look slightly different to our brains when played back correctly. Part of that is the different shutter speeds used (film tends to have a bit more blur than video), part of that is just the rhythm of the repeating images. Couple 24 fps with shallow depth of field, and you have something that looks "Hollywood." Couple 30 fps with deep depth of field and you have something that looks "TV."
So we need 24, 25, and 30 fps to create all the necessary "looks" we might want worldwide. Anything less and there are looks you can't create and you'll have footage that needs some sort of conversion to play in some formats. That conversion will make your footage look a little odd compared to native footage in that format. Video conversion is a bit like image resizing: done quickly it loses some quality, doing it right is difficult and time-consuming.
Nikon doesn't have a video division to compete with or a dedicated video camera to compete with, so there's absolutely no reason why they should cripple frame rates in their products (I'd argue the others shouldn't, either, but that's another story for another day; Sony, I'm looking at you). By only providing some of the needed frame rates, you essentially leave your "video capability" without a few critical limbs.
Yet Nikon pretty much did just that until the D5100 appeared:
D90: 720/24P
D3s: 720/24P
D300s: 720/24P
D5000: 720/24P
D7000: 1080/24P, 720/24/25/30P
D3100: 1080/24P, 720/24/25/30P
D5100: 1080/24/25/30P, 720/24/25/30P
Finally, with a D5100 I can match against what I actually need in terms of video format. Of course, I lost manual control of the camera and have to jump through hoops to keep the camera from jerking ISO all over the place, but at least I can intercut to all my video projects in different formats without facing the dreaded conversion. (Yes, I know Premiere can do on-the-fly conversion, but I find that it tends to introduce artifacts.)
So, let's all carefully watch Nikon's next DSLR announcements. If they have the same (or better) video formats as the D5100, the frame rate confusion up through the D3100 was all about Nikon catching up to video reality. If they don't include the full set, well, Nikon is screwing with us by divvying up features amongst models.
Footnote: Why 1080 and 720? At the moment, 1080 is the choice you want to make if your final destination is the big screen or Blu-Ray (note that you'll have to switch your 30P to 60i in your video editor, though your 24/25P will be fine as is). The 1920x1080 pixel count of 1080 is essentially what we might call 2mp video (Hollywood is using 4mp and higher for digital productions these days, but there has been a lot of 2mp that's made it to the big screen). The 1280x720 pixel count of 720 is about half the pixel count. That's perfect for bandwidth-constrained output, such as streaming on the net. Eventually the net's bandwidth will be high enough that we can use 1080 the way we do 720 today, but not yet, especially considering where mobile devices are. Thus, 1080/24P would tend to be the preferred format for going big screen and 720/24P would tend to be the format to record for streaming. For US broadcast use, some outlets want 1080/30P and some 720/30P (though you can usually record 1080/30P and let the 720/30P outlets downconvert). For Blu-Ray output, 1080/24P or 1080/50i/60i would be the preferred formats, and again, you can get the 60i from most editors if you record 30P (and some camera video formats actually put 30P in a 60i container anyway).