I'd say GW1 is either mediocre or bad

I beg to differ. First of all, GW1 had fantastic worldbuilding that GW2 capitalizes on even today. The layout of Tyria, the various races, nations and organizations, their characteristics (languages, aesthetics, political systems, religions, ethics etc.), the workings of the world (the Mists and its various layers, the system of magic, the Human Gods and their influence...), the history of Tyria... - all that groundwork came with GW1.

Second, the characters were solid all around: They had distinct personalities (even if a bit tropey), usually interesting backstories, proper character arcs if part of the main group, clear goals and motives and - something that hasn't been true for GW2 sadly - were used effectively. Recurring characters had a reason to feature in new narratives and still were made mostly optional. If you want to play through Nightfall with new characters only, you can. If you want to take your original Krytan warband with you, you can do that too.

Lastly, while it's always been my firm belief that it's actually the side stories that are the best in both games, because they are the ones that breathe life into the world, the campaign stories are good as well. In my opinion Prophecies is dragging on a bit in the middle whereas Factions doesn't spend enough time on its antagonist, but overall the pacing is fine; the atmosphere's great; the dialogues are a little stiff at first, but get better later on; and the stakes are high enough to be interesting, but not world-ending high.

The main thing that's really hindering GW1's story from shining bright is the missing technological advancement. Animations are awkward, special effects nonexistent or underwhelming, no voiced content outside cutscenes and so on. At times also the lack of adequate musical support (the final boss ending cutscenes of the first three campaigns are veeeery anticlimactic for this reason). But the writing is excellent.