I've been against Twilight becoming an alicorn from the start and that hasn't changed, but now that's it actually been confirmed, I'm not going to go all alarmist and freak out... just yet. Considering the writers' track records, I'll give them the benefit of the doubt until the infamous episode finally airs and we'll go from there.
I can understand the reasoning of the title of "princess" being earned based on merit in Equestria rather than a hereditary title and how this lesson about a person's efforts paying off is a nice lesson for children. I'm not against alicorn Twilight in principle, but I don't like the implications that follow from it. There's no dancing around the fact that her ascension, by definition, elevates her above her friends. While Twilight has always been the de facto leader, the other ponies differ to her because of her natural charisma and abilities to plan, lead, and delegate, not because she was a "princess".
Becoming an alicorn is going to change the dynamics of the Mare 6 in some way, shape, or form and I can't see how it couldn't. Being a Princess is a big deal in Equestria. Let's say your friend at work suddenly gets a promotion and is put in charge of your department. Sure, you'll be happy for him or her and yeah he or she will still be the same person, but socially (if we view the workplace as a mini society), they will be above you and it would be inappropriate to behave the same way you did with them as when you were equals. Privately, you may both wish it was otherwise, but that does little to change reality. No matter what history you may have had together, or how close you were, there will be a fundamental shift in your relationship. Pretending otherwise would be plain denial.
Twilight may still be the same socially awkward, obsessive compulsive bookworm we all know and love, but she's going to be something more than just the Ponyville librarian from now on. That "something more" is going to set her apart from the rest of the Mane 6. Compared to the others, she'll be "special". Sure, she's always had uncommon skill and power in wielding magic, but she only became such an exceptional individual through diligent study and hard work.
Being talented is one thing. Turning into the same race of beings that dictate Equestria's cycle of day and night and have lived over 1,000 years without visibly aging a day is another. Granted, very little lore regarding alicorns has ever explicitly been exposed in the show and most prevailing notions are the result of fanon, and that's the only reason I'm not gnashing my teeth over Twilight's imminent transformation. If the writers can expand on the nature of what an alicorn exactly is and what sets them apart from other ponies, there may be a chance to avoid derailing everything that made me like MLP: FiM in the first place.
I don't want her relationship with her friends to change and I'm just afraid that her new status and title is going to do just that: change things, and not for the better. I just really, really hope those fears don't come to fruition.