This explanation is mostly for newer Guardians.
1- Burning is a lot rarer now.
You used to heal for 500k to 650k, you don't anymore because DH is irrelevant.
Pure of Voice, when you shout convert a condition to a boon converts burning into Aegis.
Without DH to apply burning to your team, your team no longer randomly blocks everything & heals each time burn is converted to aegis.
2- Stand your Ground was a huge sustain tool
Stand your ground was one of your biggest self-sustain skills. Altruistic healing (Heal yourself when you grant a boon to an ally) healed you for each stack of a boon applied to all allies you apply it to.
So let's say in a 4vx, you give a 5x Stability and Retal to 3 other teammates. That's a straight-up 1,680 self-heal. 1960 if each teammate also had a condition on them.
3- Revs got nerfed**
Mesmers and Thieves used to be strictly outclassed by Revs, which is awful for Guardians because of the number of boonrips mesmers & thieves have access to. The more thieves + mesmers there are in games, the more likely you are to get your signet interrupted and have a bad time.
Hope this will help clear up to less experienced players why their class feels way less impactful now despite minor nerfs.