How to submit things that get accepted
I've done more than 16,000 reviews over the years so I've seen a lot of really great stuff and a whole boatload of garbage. Here's what I've learned about how to make high-quality submissions so that they have a good chance of being approved.
First and foremost, make sure the thing you're submitting is currently eligible. You can't use existing things in the games as a guideline since the rules are constantly evolving. Read the most recent AMAs and any new documentation that Niantic has published.
Now, pretend that you're making a glossy brochure for the submission. You want it to have visual appeal since people respond to that at a subconscious level, so start with a high quality photo. Taking your images at the right time of day so that the submission is well lit. Hold your camera level, and you probably want it to be parallel to the front of the thing you're submitting. Choose the best angle so that the submission is prominent, and that it's very easy to recognize at a quick glance. Be careful of cluttered backgrounds. Don't be afraid to use an unusual angle if it will present your submission well... I've occasionally had great luck by getting down low and **** upward so that the sky is the background. This can work really well for sculptures.
For the rest of the submission you need to be a salesperson and an educator-- convince reviewers that it's a worthwhile submission, and make it easy for them to approve it. Start by making your title both descriptive and interesting. "Playground" is bad. "John Smith Memorial Park Playground" is good. If the playground has a special theme or something quirky about it play off of that. "Magical Dragon and Castle Playground" would be fun and interesting.
Use your description to start selling. "Popular coffee shop" is boring. Use the description to paint a picture for the reviewer: "This quirky coffee shop feels like the living room you wish you had. The tables are great for work or study, or sink into one of the sofas with a book from the shop's 2000-volume lending library." If you're submitting John Brown Memorial Plaque then do something like this as the description: "John Brown was the founder of Flufferton, and served as mayor from 1891 to 1907. He also founded and ran the Flufferton Orphanage and Bakery."
Your supporting text is very important! The more you can pre-digest the submission for reviewers the less work they have to do and the more likely it is that your submission will be expected. Don't expect reviewers to look things up, because most won't. "Memorial plaque for John Brown ill get you a quick rejection. Try this: "Every year on May 19 the citizens of Flufferton don pinstripes and bowler hats and stage a John Brown Parade to celebrate his legacy. The cake party afterward is legendary."
Another option for supporting text is to help your reviewers find the correct location. "This statue is under tree cover but can be seen from street view from the north side. It is ten meters west of the front door." Think about what sort of information would be most helpful to reviewers and present it here.
Make sure your supporting photo shows the submission with enough context around it so that reviewers can find it. It's pretty common for things like urban artwork to be too new for street view, for example. Step back and take a wider shot of the area. Make sure the thing you're submitting is clearly visible, but show enough of the area around it so that reviewers can use street/satellite view to confirm the location. If you're submitting something at the corner of State and Main then having the street sign in the image might help. Are the buildings near your submission visually unique? Include them to provide context.
These are Hosette's tips for getting stuff approved. Make your submission a glossy brochure, and hand the most useful and interesting information to reviewers on a silver platter so they don't have to work as hard. I'm always thrilled when I find submissions like this because they're so much easier to review, but they're also pretty rare.