I recently upgraded My Dell 27 Inch monitor for an LG 42 inch 1080p TV. This did not make my wife very happy but then again she doesn't Borderland 2. It seems to work fine with all the extra stuff turned off but there is some game lag.
What you're experiencing is actually motion input lag, which is usually caused by the image processing technology in your TV. This can include the higher refresh rates, motion or edge smoothing and other features. This takes a few microseconds to happen, and adds "lag" to the game.
So you might press W, A, S, or D and it will feel like it's taking longer for it to respond. When moving the mouse, it will feel like there's lag on the screen - but it's just lag on your input method. This is why controllers on consoles feel so much slower to operate in a first-person shooter than a 60/120Hz monitor on PC using a mouse.
The response time on an LCD monitor is much better than most TVs because of this - so to go from your Dell 27-inch monitor to the LG 42-inch HDTV, it's actually a step backward in quality (if quality is lag/response), but an upgrade in physical screen size.
This is why I personally use 120Hz-based monitors for my first-person shooter gaming. I have the latest Samsung 55-inch Smart TV, and while it's a lot better than most TVs out there in terms of input lag, it doesn't even begin to compare to a proper LCD monitor.
One of our great readers Justin, made a very good point about enabling a "Game" mode on the TV. Jeff replied saying that this, along with some other tweaks he made, made a huge improvement in his experience.