What's off? Well, I'll give a short critique a go, but don't take any of it as me being cruel, I'm just trying to help! It is a nice image but I can see some things being out of place:

- Head/body proportions and overall pony sizes: These change throughout the image (e.g. Dash's head compared to Rarity's, Twilight's body compared to everypony else's). It's the type of problem I normally get if I haven't spent enough time on the initial sketches/lineart.
- Head shapes: All in slightly different styles. For example, Fluttershy has small eyes facing forward near the snout whilst Twilight has large eyes at an angle almost half way between her ears and snout. Now, Twilight has more slanted eyes in the series, but so does Rarity and here she has forward-facing eyes about half way between horn and snout. Rarity's eyes are also close together than Applejack's. (For personal opinion: I think Fluttershy and Pinkie's head styles look the best. Fluttershy's eyes are further forward but they're both slightly more realistic versions of the show: Smaller eyes but roughly the same head shape.)
- Snout angles: Some of these are off... Mostly just the nostril placement (Twilight and Rarity), it makes it look like their nostrils are almost on the top of their muzzles.
- Rarity's throat looks really flat. It should be brighter where the neck comes forward. I'll use this photo of a horse to show what I mean (the lighting's different of course but the angle's about the same and it shows the throat structure somewhat): [link] Obviously this pic isn't after complete horse realism but still.
- Twilight's horn seems a bit far off from her forehead.
- Rainbow Dash's snout is too large (especially compared to those of Rarity and Applejack)
- Rarity's outreached leg doesn't look as angular as it could, making it look floppy around the "knee" (in horse-talk, or "wrist" if you prefer the human anatomy equivalent): [link] [link] (Horse and deer since I'm not sure which you were favouring, what with her being a unicorn)
- Fluttershy's back seems to have partially absorbed that cloth. I think her back should be generally brighter apart from below the cloth, which should match her back's shading a little closer (at the moment she could have a dark patch of fur and the cloth's on top of it).
- Pegasus wings: I think they'd benefit from being more realistic. Wings are a bit of a pain to do but the best tutorial I know of is on drawing wings: [link] (You don't have to move towards a realistic style if you don't want to but wings can be very striking when realistic, and at any rate the tutorial's a fun read)
- River ripples: I feel these should give a better sense of direction... Having trouble finding an image to illustrate what I mean but I think it would have to be a strange river to have ripples around the unicorns' hooves like that (unless they've only just dipped them in but I got the impression they're just lazily letting them touch the surface). I think the ripples should be almost non-existent (if it's a very gentle river) or that they should leave a slight trail in the direction the river's flowing (Pinkie's left hoof does somewhat). It's possible I've misread your intention on this point though, sorry if that's the case.
- Hair: You've given most of the ponies more realistic hair but not so much Applejack's fringe or Rainbow Dash's mane/tail. The little tufts coming out look fine for a cartoon style but feel out of place here, especially on Dash's tail since it's silhouetted against the background.
- Also, the right side of the background may be a little too simplistic (leaves looking like a green cloud).

Now some highlights!
- It's a really vibrant (whilst not jarring) use of colour
- Generally you've done a great job of the shading!
- Taken in 1 at a time the ponies all look good (particularly Fluttershy, Applejack and especially Pinkie)
- It's a really pleasant scene! By an alcohol river in a relaxed manner: It's not done enough! I mean, I like "EVERYONE GET DRUNK!" take on the concept too, but the casual side's lacking.
- The head decorations really work for AJ and Pinkie!
- Rarity's tiara has great detail.
- I like how you've done Twilight's mane. And Pinkie's, but then I think it's clear that I like everything you've done with Pinkie.
- The background on the left looks good (the vines hanging from the tree and the clouds around the mountain; these are nice touches)!

Overall it's a really pleasant pic, just a few things off. I'd recommend more preliminary sketches of the characters solo (to decide on body proportions/face type/etc) and longer on the lineart to make sure it all matches up. Good luck for the future, I'm sure you'll surpass this (already very good) level fairly quickly!

Awesome stuff; just had to write something.

VISION: You did well in simply capturing the atmosphere/general mood of the piece. It's a summer/spring afternoon, their expressions bringing a sort of comfort and satisfaction to the image, making the viewer feel the same, in a way. Especially Pinkie Pie and Fluttershy, who seem to be the most occupied with this feeling.

ORIGINALITY: Obviously, this is fanart, so don't worry too much about originality. I like the nostalgia this brought on, though, as it made me recall my favorite animation out of Fantasia, involving the ponies and the humans, ponies and satyrs all having one giant everlasting party. But I still can't give five stars for originality simply because it is fanart, and that I've seen the concept used in a popular film.

TECHNIQUE: Your style brings a less-shocking style of realism to the image (IE, not one that relies heavily on real-life anatomy) in the way that you kept the short snouts of the characters, allowing the ponies to retain a bit of their charm while still looking different from the general style of the show. Great detail in the wine and fruit. Shading is excellent, although the only qualm I have about that is that there seems to be no shadow for Twilight and Pinkie's "togas." The fruits' detail is amazing, particularly the grapes and the watermelon. I actually like that the background is soft and out of focus, as it keeps the attention on the characters in the foreground.

IMPACT: Again, the aforementioned nostalgia from the first Fantasia movie adds to the impact. The general carefree attitude that each of these ponies is expressing is a nice feeling overall, and when added to the nostalgia, makes a wonderful combo.

Great work; looking forward to more.