Half Life 2 Deathmatch was the greatest FPS multiplayer ever made.  It was
the first multiplayer that used physics as an integral part of the game play. 
The gravity gun made it unique among games in that it allowed the player to
pick up objects in the environment and use them as weapons, instead of just
having guns.  Sadly, the game never gained the popularity of other FPS multiplayers because it had a large learning curve and it took a long time
to develop the skill level needed to be a competitive player.  However, it was
the continual challenge of learning the game and working to get better
that kept many of us coming back for years.  It never got boring so long as
there were challenges to be met and things to learn and get better at. 
For those that learned, the game play was fast
and dynamic and always a challenge. 

Lack of continued support from Valve to fix the bugs in the game and to
update it began to take its toll.  Many long time players left for games with
fewer problems and some left to follow other life challenges.  For a while
they were replaced by newer blood eager to learn and advance in the game,
but many of them couldn't deal with what it took to succeed despite the efforts
of a few people that tried to teach them.  The game became a wasteland of
killbox, low gravity, low skill servers where advanced techniques were
deemed as scripts and hacks and the skilled player was not welcome.
 
In the end, the fall of CAL, the only league the game had left in North
America, was what ultimately did in this great game.  There was no longer
anything to be challenged for, no reason to learn the skills that gave the game
its depth.  What remained of the top players and clans faded away as newer
games with newer graphics and relatively easier play became commonplace.

For many of us, HL2DM will be the greatest FPS game ever made no
matter what new games come out and what new graphics. 
There will never be another game like it again.