Ear Worm: Given that Haruka, like other NeoRomance titles, spawns enormous amounts of character song CDs and live concerts, there are plenty of them. Early OAVs also have catchy J-Pop theme songs, but in some cases it's advisable not to listen to the lyrics too closely. There's even at least one song ("Harari, Hirari", the Ending Theme for the Maihitoyo movie), which, at least within the context it is used in, can function as an Ear Worm and a Tear Jerkerat the same time.
Foe Yay: Akane clearly falls for Akuram after seeing his face, which causes her a bit of trouble throughout the story.
Growing the Beard: Early stories, even though exhibiting a nice amount of work done, might still come off as a little generic (hence all the Fushigi Yuugi comparisons). Then Haruka 3 picked up a specific historical event complete with Public Domain Characters, made the main character more of an Action Girl and played with some of the plot elements inherited from its predecessors. It is by far the most popular entry in the series.
Ho Yay: The Hachiyou, to various degrees. Naturally, fanfiction tends to, ahem, exaggerate this...
Ho Yay Shipping: Goes without saying — eight pretty guys already bonded in pairs is every Yaoi Fangirl's dream. Particularly popular 'ships among the first game's cast are the Byakko pair (Takamichi/Tomomasa) and the Genbu pair (Eisen/Yasuaki). The former two do exhibit enough generic Ho Yay, but the latter pair provides additional interest due to the possibility of exploring Yasuaki's emotional development — even though canonically they only develop into real friends in the anime; manga!Eisen seems to think that Yasuaki is scary.
Tainted by the Preview: Haruka 5 initially caused a lot of disappointment due to changing the voice cast and skipping over to the Bakumatsu Period, to the point that some fans have voiced an opinion that it should have been a new series rather than a continuation (by analogy with Neo Angelique). Of course, as more info was revealed, things turned out to be a lot less dramatic.
They Copied It, So It Sucks!: So. Very. Much. Was really worth mentioning right on the main page due to sheer persistency, as one of the first things people outside Japan learn about Haruka is that it's Fushigi Yuugi with Serial Numbers Filed Off. Nevermind that this opinion is often only based on the manga/anime adaptation of the first game, and the series has been Growing the Beard for quite a long time since.
The Woobie: Many of the Hachiyou have tragic backstories in one way or another. Quite understandable as this character type is intended to be liked by the audience — which is sort of the point here since Haruka is a Romance Game series and has to give its characters traits that could attract the player.
The end of Hachiyou Shou TV episode seven, particularly when Yorihisa hugs Akane and asks her not to look at him for a while, after he assists his childhood friend in performing Suicide by Cop. Yorihisa's Image Song as the Ending Theme makes it even worse.
The climax and the Bittersweet Ending of the Maihitoyo movie. After Suefumi turns into an onryou, Akane can't bring herself to use her powers on him. Tenma orders Yasuaki to do the job, but Yasuaki refuses on the account that Akane will cry, so the Hachiyou instead convince Akane that only she can save Suefumi. Akane then manages to get Suefumi to turn back into his normal form and notes with a Tearful Smile that she can finally call him by his name — and does this over and over again, ending up crying in his arms. After that, she purifies him, freeing his spirit, and the movie cuts to its Ear Worm ending song, which is pretty sad on its own and only adds to the impression.