There could be a few differences. Despite both being LCD there are many types of LCDs.
Computer monitors are mostly TN they have fast response time but crappy color and terrible viewing angle.
Since TVs must be good for viewing towards the sides, TV panels are some type of VA. Better color and viewing angle but lower response time. A 30" monitor would have more pixels giving a sharper imagine while a 30" TV would have simply bigger pixels giving a less sharp image so TVs are not that great for close viewing.
Connectors are something to consider. TVs usually have lots of connectors for your playstation, xbox, and tons of other stuff you might want to plug in. Monitors usually have limited connectors, usually 1 of each so your choices are quite limited if you want a home theater plugged into it.
Input lag is something to consider. The time it takes for the monitor to display the image after recieving the information from your GFX card. It can be as high as 30ms for some displays. Usually TV video processors are slower and if the TV has 120hz, it would be even slower. Monitors would have input lag as well but usually not as much as TV. The only displays that could be said to be free of input lag is probably the IPS panels that have no scaler or processors what so ever, so no options, no on screen displaay and only 1 input.
As far as connectors go, It would be nice to be able to connect my PC, XBOX 360, and cable to a single screen, but if a LCD TV is really going to sacrifice that much clarity, I'll probably end up having both a Monitor and a HDTV.
One issue some people have with using an HDTV as a computer display is the desktop extends beyond the HDTV screen even though the desktop is set to 1920 x 1080. It happened with my HDTV (LG 47LH90; IPS panel) and it was quickly resolved by going into the display options and choosing "Just Scan" in the Aspect Ratio option. It might be called something else in other brands if there is such an option.
I have not used my HDTV for gaming since I don't play console games and most of the games I play are rather keyboard intensive so I rather play in front of my PC. If you do want to game on the HDTV, then always set it 60Hz mode; 120Hz and 240Hz modes increases input lag due to internal video processing.
I have my HTPC connected to my HDTV to watch movies.
An HDTV is just like an LCD monitor; regardless of what panel technology is in it (TN, *VA, IPS) there is always some level of input lag.