I really like this, and not just because I'm a fan of TwiDash. It's cute, warm and really gives an impression of two ponies who love each other a lot. The blurry lights in the background help give the impression of a night-time scene with various light sources in the distance, without making the piece feel cluttered. The rain effects look nice.
My one major criticism is the "facial features visible through hair" that you have going on with the left eyes of both Rainbow Dash and Twilight. Transparent bangs irritate me.
You may also want to darken and desaturate the puddle a little, as it seems too vibrant in comparison to the rest of their surroundings.
Let me first start off by saying that the wet mane, even if it is over used, looks amazing. All of the focus of the image is on the two ponies in the rain due to the simplicity of the background and the vibrant colors of Rainbow and Twi. One of the things that really catches my eye is how you did the reflection in the puddle, very nice.
Normally I don't appreciate the transparent bangs, but the subtlety of it makes it fit with the image and doesn't take away from the overall quality. The mist effect from the rain hitting their bodies is duly noted and brings the piece to life. Their expressions and emotions are very well illustrated and push the image forward. Someone could really write a story just from what they are seeing here in this still shot.
In the end, everything comes together really nicely and the only thing that really brings down my overall score is that the wet mane crazy is starting to get old.
I must ask, you don't mind if I use this as a picture for a fanfic I'm writing, do you? It's titled "Through Wind and Rain" and this picture is wonderful. It is a TwiDash shipping fic, but I would like your permission before I upload it. Of course, if you'd like, I will provide a source for this picture. It's magnificent and I think deserves plenty of publicity. Please get back to me quick at: Tyler2Davis@gmail.com or on my Skype: Tyler Davis. If you can't find my Skype using that, type in the email I used which is: email@example.com. This picture is literally what I need and I'd appreciate it if you could get back to me as quick as possible...
I'm still am puzzled about one thing and I can't believe I'm stressing out about it, but What would you talk about when sitting in a puddle while it raining? (Other than silly jokes like Wonderful Weather were having) But even so, wonderful Picture of those two sexy ponies