1) Large lens diameter to let light in for the imaging chips to capture. 72mm diameter or larger.
2) Large imaging chip - usually a 3CCD or 3CMOS imaging chip array (1 chip for each primary color - Red, Green, Blue). 1/3" or larger.
These first two items deal with processing the light and images. Larger = better low light behavior. Large lenses and imaging chips are expensive and - well - large.
3) Audio connectivity generally is a secure/robust dual XLR connection (left and right audio) rather than a 1/8" stereo audio input. Many times, there is an auxilliary 1/8" input, but that is not a primary connection. XLR mics are typically more robust, too.
4) Manual controls on the outside of the camcorder... zoom, focus, white balance, audio gain (preferably 1 left, 1 right, but a single control for both channels is acceptable), neutral density filter selection, exposure/iris.
5) Low compression video capture onto the storage media. The actual media - commonly digital tape - does not matter. The format of the video captured is important. High compression = reduced video quality. Low compression = improved video quality. DV/HDV, DVCPro/DVCProHD, XF, XDCAM/HDCAM, Digital Beta are examples of pro-grade low compression formats.
6) There is typically a way to securely attach an add-on lens using bayonet or screw-mounting. This should not be confused with "interchangeable" lens systems that dSLR/SLR cameras use where the whole lens comes off to be replaed by another lens.
There are exceptions to the above, but for the most part, these are the obvious items that move a camcorder into the higher end.
The low end of consumer camcorders generally have small lenses (37mm-47mm diameter) and small single imaging chip (1/6"). Low light behavior is poor. Little or no manual audio gain control, no manual controls on the outside of the camcorder (difficult to get to or use).
The mid and high end of consumer camcorders have larger lenses (58mm) and single imaging chip (1/3"). They might have manual audio control and maybe a 1/8" stereo audio input.
All flash memory and hard disc drive consumer cams these days use high compression AVCHD. AVCHD is an OK technology but not for high compression capture. It can be a challenge to edit.
Prosumers generally have the larger lenses and imaging chip systems but with a 1/8" audio input rather than dual XLR inputs. This is resolved using an XLR adapter (juicedLink, BeachTek).
Long term storage is poorly defined for any flash memory or hard disc drive recording media. Digital tape is an acceptable long term storage format.
There's lots more, but this is the obvious stuff...
And how are professional camcorders different from prosumer/consumer camcorders?