Glenn represents hopes and beliefs in people, he always represented hope.
Abe went out satisfied and his story at its close, but Glenn's was cut short. I have no idea what Abraham's death meant to them, but Glenn's meant that all Rick's hopes and expectations along with everyone elses was killed before them.
Abraham represents strength, courage and perseverance. He was the biggest and toughest member of Rick's group and he was a military sergeant. He became a vital member to the group because of that. And with his death, the group are at a certain vulnerability and they will need to recruit more manpower to fill in the void that Abraham's death has made.