Defenders are some of the best/most reliable crowd control, a defender can lock down an area making it encounters significantly easier.
The players that like to count their kills will probably not enjoy defenders, however those who enjoy controlling the flow of battle, and the tactical positioning of themselves will find defenders a very rewarding and fun role to play. Defenders still do solid damage, a little less then an striker, but they hold up better, and get more opportunities to mess up the enemy.
Even at first level the fighter moves enemies around, forcing them into clusters for scorching burst, or separating monsters that benefit from being near each other. in some ways they hold similarities with the warlord (directing movement ect) but while the warlord makes your party better, the fighter makes their party worse.
The paladin is slightly different, he seems to pretty much be single target tank, he grabs a guy and duels that guy, however he has some party benefit abilities so he helps via heals/temp HP for his party while he holds one guy on himself.
Wizards blow some things up, they are more blastery (not a word, w/e) then they were, which is good because spells like glitterdust can simply end encounters, which isn't so much fun for everyone else.
Some players feel that the number of things they kill is their total worth, while others realize that just doing damage and not hampering the opponents leaves you dead in tactical situations (ie the monster is smart enough to ignore the guys in full plate, and kill the guys in cloth/leather.)