To be honest, I've tought about switching to Nikon too. It almost drives me insane that it seems like Canon isn't doing anything about their (heavily) inferior sensors. With every release of a new DSLR you see Canon getting beated to death by Nikon and Sony when it comes down to DR. The same goes for noise performance at high ISO when you compare similar sensors (megapixels & sensor size) e.g. D7000 vs 7D.
It's also a bit of a sentimental thing. While I don't really have anything against Nikon, I find their bodies rather ugly (shape and red triangle) and I also find it harder to understand their lense lineup compared to the lens lineup from Canon (e.g. I can't tell which lenses are really top of the bill, except for their 70-200 F/2.8 VRII).
It is harder to distinguish between Nikon's lenses because they classify by the type of lens (basically how new it is) rather than the perceived quality, such as Canon's L line. AF-D lenses are the older type - they have aperture rings and focus via an in-body motor-driven screw. AF-S G lenses are the newest and have Nikon's ultrasonic motor (SWM - silent wave motor) and do not have an aperture ring. There are some that straddle the line between the two, but those are the two main groups.
Top of the line Nikon lenses have a gold ring around the front of the barrel: 14-24, 24-70, 70-200, 24/1.4, 35/1.4, 85/1.4. There might be a few more but you get the idea. These lenses are quite similar to the Canon equivalents, with the exception of the 85 being f/1.4 instead of f/1.2. Nikon also doesn't have a 50L competitor; the 50/1.4G is slightly better optically and much better built than Canon's 50/1.4, but nobody would ever confuse it with a 50L.
One step below that is the new f/1.8 lenses: 28/1.8G, 50/1.8G, 85/1.8G, which are really phenomenal and quite reasonably priced. The 85 in particular is my favorite lens and performs extremely well for $500.
Also note that Nikon just today announced a 70-200/4 with a new 5-stop VR (IS for Canon folks). I'm excited! This is one of the two lenses that Nikon hasn't had, along with a good 135/2, that almost made me not switch. Looks like it's one down, one to go
I know the difference between AF-D and AF-S lenses, but beyond that I still find it hard. Also the fastest focusing Canon lenses are the ones with an Ultrasonic Motor (USM). Which are the fastest ones in the Nikon lens lineup? Lenses with a SWM? With Canon it's much easier. Most of the time L glass = high-end glass. And besides that there are some gems like e.g. the EF-S 17-55mm F/2.8 IS or the EF 85mm F/1.8. Yeah I've heard about the Nikon 70-200mm F/4. If it's as good as the Canon variant, they have 'disabled' one of the advantages of Canon. However there are still no (decent) equivalents of the 65mm MP-E and the various TS-E lenses. Nikon also doesn't have a pancake lens?
As far as focusing speed, you have to look up reviews of individual lenses, which are pretty easy to find. There are focusing speed demos of lots of lenses on youtube. Even with SWM, the speed varies greatly. For example, the 50/1.8 focuses much faster than the 1.4 despite them both being SWM. EDIT: some of the D lenses focus more quickly than their newer G counterparts, especially on pro bodies due to their powerful in-body motors. These focus very noisily and are considered by some to be quicker but less accurate. I don't own any D lenses and really wouldn't consider buying one due to the advancements in coatings and optical design found in the new G's.
Nikon has an "L"-like designation too - the gold ring. It's really easy to tell the build quality difference between gold and non-gold lenses.
The new 85/1.8G is better in every way than the Canon 85/1.8, though it is slightly more expensive. It focuses just as quickly while having much better bokeh and is extremely sharp all the way across full frame by f/2.8 (bitingly sharp in the center wide open).
Nikon's 17-55 f/2.8 is not as good as Canon's - it doesn't have IS but does have gold ring build quality; it is a slightly smaller 24-70. Nikonrumors reported that Nikon will soon release a new set of DX lenses, so presumably an updated 17-55 would be among those.
There is no pancake but the 50/1.8G is extremely good. Nikon also has an amazing 14-24 and a quite good 16-35/4 VR, along with the new G primes that Canon hasn't answered yet. Both systems are fantastic, and each one has its perks that the other can't quite match. You really do have to research individual lenses to see if they appeal to you; it's no different from Canon in this regard.
As for the 135 DC, I have read very mixed things. It's older, very heavy, and very expensive, and that alone dissuades me from considering it. A 135/1.8 VR has been long rumored, so we'll see if that ever pans out. In the meantime, my 85/1.8G is phenomenal and very affordable.