Not a native speaker (nor an expert), but I've studied the 하더라/했더라 issue enough that I should be able to explain it. ㅎㅎ The "더" communicates the nuance that you've experienced this thing personally (or learned about it). It's not something that everybody knows or that you have known for a long time. It apparently sounds very strange to use past tense + 더라 with an adjective (descriptive verb, whatever you want to call it). "해운대해변에 가보니까 좋았더라!" X "--- 좋더라!" O You're expressing a snapshot of your experience, so it makes sense (to native speakers at least!) to use present tense.
I've found it hard to wrap my head around this issue, so I thought it might be helpful to share my conclusions with a fellow learner. 희섭 씨 covered the rest perfectly.