"Traditional" LCDs use a backlight source that is a CCFL (fluorescent). The LED LCDs use a backlight source that is well an LED. That's the technical difference between the two.
The LED light source provides your LCD TV blacks, colors, and contrast that are far superior. It would push the TV a lot closer to Plasma like levels (not quite, but very respectable). But you have to pay a price premium for this feature right now.
Because LED sets are supposed to be 'high end' sets, they also will likely come with 120Hz or 240Hz refresh rates. This helps to overcome the inherent flaw of LCDs. LCDs are bad a fast motion. They suffer from motion blur (ghost trails). So hence the higher refresh rates. But again, its part of the price premium you pay for these sets.
As you're looking at 46" you are realistically in the mix for having to care about the color, contrast, and blacks of your TV (and the refresh rate). As you break 40" (but moreso 50") those things matter a lot more. So there is potentially image quality value in going LED.
But you could save money and just go with a Panasonic or Samsung Plasma. It would give you better image than even an LED LCD. And more importantly it would cost a whole lot less.
But, all that being said, people have different sensitivities to these things. So you need to go into a store and do some direct comparisons for yourself. You're the one watching your TV, not people on the Internet, so it needs to be suitable for your eyes. At the end of the day, if the image you see looks great to you, then the tech specs are meaningless. You're buying this TV to entertain yourself, not the Internet.