Howard Fineman has expressed, better than I did, the point about the biggest, longest-enduring casualty of today's mass shooting:

The End Of Access (Howard Fineman, 1-8-2011)

Excerpts:
"WASHINGTON -- The shooting of Rep. Gabrielle Giffords is a watershed event in many ways, some of which we cannot yet know, but one of the clearest and simplest is this: Congress and its members are about to be permanently quarantined, physically isolated, from the people it and they represent.

Thirty years ago, there was no such thing as security on Capitol Hill or for members. Members of the public were free to roam the halls, and police presence was practically invisible ... and even the leadership rarely had any form of protection.

The Hill was the very model of the People's Place -- and in that respect it was an inspirational symbol of our democracy.

Congress began to close in on itself in 1983. [A bomb explosion in 1993, a shooting in 1998, the 9/11 attacks and an anthrax attack in 2003 each led to tighter security measures & greater restriction of access to public officials & buildings.]

...

New even more restrictive rules are now inevitable. It's even possible that the general public will be banned from the hallways of the Capitol Complex, at least at certain times and under certain circumstances.

As for personal protection, that is likely to be increased substantially. For the last year or two, some House members and senators have had unpublicized but substantial security details dispatched to their side when deemed warranted.

...

But rather than have extensive details for each member, the members are likely to change their behavior -- which means they will stay behind closed doors here in DC and in their home districts and states.

... "

This is especially likely, and especially sad, because today's small, informal "Congress On Your Corner" meeting between Rep. Gabrielle Giffords and a fairly small gathering of her constituents was a continuation of this practice which she began back in 2007.

From the press release from Gifford's office re today's gathering:

"Congress on Your Corner" allows residents of Arizona's 8th Congressional District to meet their congresswoman one-on-one and discuss with her any issue, concern or problem involving the federal government.

Giffords has hosted numerous "Congress on Your Corner" events since taking office in January 2007. As in the past, the congresswoman's staff will be available to assist constituents."