A Midsummer Night's Dream/Headscratchers

Demetrius is essentially fed a date rape drug and forced to marry his stalker. How is that a happy ending?

"If we spirits have offended, think but this and all is mended: That you did but slumber here, as these visions did appear." Translation: If you don't like it, it was All Just a Dream.
When my school did this, the undpoing of the love spell involved a surreal Disney Acid Sequence puppet dance, so that it was unclear whether Demetrius was still under the spell or not, or at least distracted the audience enough so that they didn't notice.
The entire play revolves around the story of faeries casting magic spells on humans, and the love-potion is brewed from Cupid's arrow. It symbolizes the apparently random way humans fall in love with each other - so if you're looking for a tragic ending to the story, it would be that love is a completely random and arbitrary force that will get you in a lot of trouble, and you might just fall out of it just as easily.
Poetic justice since he tried to force another girl he was essentially stalking to marry him.
Shakespeare was ahead of his time when it came to Estrogen Brigade Fan Service?
99% of the time, if something bothers you in Shakespeare than it was supposed to. The resolution of love in MSND is something very difficult to decode, since it does rely heavily on taking away free will, but love in Shakespeare is often a matter of youth, a time of fiery conclusion, finding it's way to settle and compromise into adulthood, rather than finding things resolve in perfect and happy ways. There's a reason that Romeo and Juliet were too cute to live. Stll, while this compromise often happens, we're not necessarily supposed to be entirely cool with it.
On the other hand, the feeding of the potion to Demetrius could be read as not a "love potion" but of a disenchantment, which clears his eyes and lets him see the truth of Helena's affection, whom he actually has chemistry with rather than Hermia, to whom he's an ass and has none. Furthermore, if his love for Helena is enchanted, it puts the choice in her hands, who, while previously disenfranchised by her love, had matured considerably over the play, and while still in love, had gained some strength and dignity.
I can't feel too much sympathy for him considering he was, you know, threatening to rape her in the woods. Serves him right for using and abusing her to begin with.
- Eh, I guess it depends on the production you see, but I always saw those two lines as Empty Threats to try to get rid of her, ones which horribly backfire when she Jumped At the Call.
It's more of a marrying his stalker ex-girlfriend. Demetrius was courting Helena (and apparently quite happy with the arrangement) until he dumped her for Hermia. The love potion could just have reminded him of what he and Helena used to have.
The simple explanation is that Demetrius is just as much a victim of the social pressures of the time as Lysander and Hermia. He doesn't really love Hermia; he just knows it's a good marriage, even if an arranged one, and respects the wishes of her father. It's only when the love juice is applied that he experiences real love for the first time. You'll notice that until that point he doesn't actually refer to Hermia with any affection other than that of an object which rightfully belongs to him rather than Lysander.
- But he does. Like, all the time. "Relent, sweet Hermia..." (I.i.91), "And here am I, and wood within this wood, / Because I cannot meet my Hermia" (II.i.192-193), "O why rebuke you him that loves you so?" (III.ii.43). In fact, he acts as obsessive and melodramatic as Lysander does when under the love-potion's spell... hmmm.
  - In that case, then doesn't drugging him rescue him from spending the rest of his life in unrequited love anyway?
Or there's always the point that we are dealing with The Fair Folk here. They don't nessarily have Demetrius' best interests at heart.
This troper actually wrote an essay on the shifts in power play between the four lovers. Pre-love potion, Lysander and Hermia are with their true love, so Lysander treats both of them with kindness and affection, whereas Demetrius thinks only of himself and his emotions. During the love potion, it gets switched - 'Demetrius' treats both Helena and Hermia with kindness and respect, whereas Lysander doesn't give a damn about either of them, caring only for his emotions. The love potion basically grants stupidly strong desire, which was what Demetrius was feeling. When reapplied, it cancels itself out, and he acts as his true character is - someone in love with Helena who is essentially decent. So, to answer the question: Demetrius hasn't been forced to marry his stalker - you could argue he was in a self-made date rape drug that he got kicked out of thanks to the elixir.
It's unlikely the audience would have been too offended by a man being forced to marry a girl he already had sex with, as Lysander notes Demetrius did when trying to make his case that he, and not Demetrius, should marry Hermia.

It's Hermia we should be worried about. If we go with the theory Demetrius is revealing his true nature once he's potioned up, doesn't that mean Lysander is a horrible, abusive prick?

But then again, we can easily discount that theory - most especially as the 'true nature' doesn't take effect with Titania, who literally becomes blind to physical defection once she is put under.

Why does Oberon care so much about the love problems of a couple humans?

He doesn't, he's just bored.
Also, he's pissed off at Titania and wants to take it out on someone.
I Always figured that fairy morality was incomprehensible to humans, so the reason for his motives was not meant to be figured out.
I always thought it was an author saving throw. Before he's come across as a bastard at best, pervy at worst (WHY does he want that kid so badly?) I for one wouldn't appreciate it if I was forced to fall in love with a jackass (excuse incredibly lazy pun). By feeling sympathy for Helena, it shows he has a good side.
- One Alternate Character Interpretation is that Oberon and Titania are becoming more human (thereby partially explaining why they're feuding now) and part of Oberon's plots is a combination of Pet the Dog and trying to understand What Is This Thing You Call Love?. Whereas before, he and Titania slept about (as seen in their comments about Theseus and Hippolyta), now Oberon is starting to feel jealous of her attentions - the Indian boy can be seen as one of the first things they haven't shared - and he doesn't understand why he should feel this way. Helena and Demetrius come through and decides that he can at least make one couple happy. Of course, Hilarity Ensues, but that's Shakespeare for you.
Because he had already asked Puck to get the flower so that he could enchant Titania and figuring that since Puck already had the flower and was going to use it, he may as well get the pesky humans out of the forest by enchanting Demetrius with the love juice.
Aren't god-like beings usually motivated by empathy? Helena's troubles at the point of the subplot intersection are similar to Oberon's; he just knows how to fix them.
- When I was in a production of MSND, this is how it was interpreted. Oberon is impressed with the strength of Helena's love for Demetrius - who clearly wants nothing to do with her - and that she's being proactive by chasing after her man instead of waiting around for someone to chase her. Oberon has the means and the might to make a miserable person happy, so he does.
Again, he's one of The Fair Folk. His morality is complex at best.

Why does Helena immediately assume that Hermia is also making fun of her?

When Lysander and Demetrius start fawning over Helena, Hermia walks in and is immediately upset. Helena jumps to the conclusion that all three of them are conspiring to play a cruel joke on her. Why wouldn't she just assume that Lysander and Demetrius have conspired against her? Why wouldn't she have perceived that Hermia was not acting?
- Because she's tired, confused, hurt and very angry. She's not really thinking clearly.
  - Also, her self-esteem has been pretty well trampled by the time the play starts anyway.
  - And since she thought that it was obvious that the two men were playing a joke on her, she probably expected Hermia to say something like, "Stop picking on the poor girl." (except, you know, in verse and all that.) When Hermia instead treated the "obvious joke" as if it was real, Helena assumed Hermia must have been in on it.

In all the worrying about the humans, nobody worries about Titania?

Her offense is to have custody of a child her now-estranged husband wants. To get the child, he sends one of his servants to get a powerful mind-altering drug and force her to fall in love with some sort of hideous monster, with the intention of humiliating her. Then, while she's drugged out of her mind, he takes the kid.
- Faeries are just screwed up like that. *shrugs*
- This troper just wonders how long that "happy ending" for the two last. Her last line in the play is to ask her husband why she was lying on the ground with Bottom. Unless he can come up with one really convincing lie, chances are that she'll be suspicious and pick up where they left off.
- My personal theory is that, by the time the play starts, Titania and Oberon's argument over the changeling has become a matter of pride more than anything else, and when he finally takes the boy she's sort of relieved that it's over. After all, they're husband and wife. Once they reconcile and become "new in amity" they could easily become the boy's adoptive parents and raise him together.
- Also for most versions of the Fae are constantly playing games with each and changelings are often the playing pieces. Oberon won this game, Titania can have another go later. Sucks to be the boy, but for the King and Queen this is business as usual.