Suggestions have been taken by Three Rings, though they might not take the whole suggestion they take the idea and add their twist to it most of the time examples:

Guilds ---- This game once never had an option for guilds, but a group of players (SGA) back in beta suggested a guild system and ta-da we received it. SGA = first ever player-made guild.

Pets --- We have them as accessories

Bosses ---- Tier 1, Tier 2 bosses never existed at one point Vanna was not even alive =0

Heat Amp's ---- Suggested this along with many other players

Stackable Amp's --- they used this idea for Krogmo coins, but improved heat amp's overall.

Accessories --- whole idea was suggested before

A lot of ideas have been suggested to three rings not all of them will be put in-play just like most games, but the staff does actively read the suggestions and keeps it-in-mind (Keep in mind the forums don't show close to 30% of the community, the surveys and such is most likely were three rings gains more player feedback).

Subject 1
Subject 2

Notice that Subject 1 has been implemented into the game. Subject 1 received 72 replies, including a few responses from OOO.

On the other hand, Subject 2 has received 363 replies. Despite this significantly higher number, OOO has yet to respond directly to that thread (or any similar threads, as far as I know) and has merely hinted at the possibility of implementing Subject 1.

It remains unknown whether OOO truly takes things from the Suggestions Forum into account or whether some of those things simply happen to coincide with what OOO is already working on. The common belief as far as I know is that the Auction House - now something like a landmark in Haven - came out of the Suggestions Forum a long time ago, but I haven't verified that myself. Njthug, Glacies, and Dirt have provided other examples of fairly old suggestions that ended up in the game, but whose roots (to my knowledge) can't be directly linked to Suggestions Forum topics. EDIT: That is, I don't think I've seen or heard of OOO posting on topics saying explicitly that they would work on the topic on account of the topic popping up in the forum.

 
Anyway, here's how the Suggestions Forum usually works. There's basically three paths a thread can take:

1. The thread is received well by nearly everyone who looks at it. It receives a lot of positive feedback, and the original author responds well to negative feedback. As a result, the thread's content eventually grows up to a point where basically everything that could be said has been said. At this point, the thread dies out as new threads enter the Suggestions Forum. Example.
2. The thread is not received well by everyone, and in fact many people respond to it negatively. The original author fails to respond appropriately to feedback, bringing even more negativity to the thread. The original author gives up on the thread, or the community gives up on the author. The thread dies out. Example.
3. The thread is received well by many people, but someone responds with a long, detailed post whose fundamental ideas appear to discredit any reasoning behind the original topic. Most people try to ignore the long post and continue supporting the topic without any proper reasoning, but it soon becomes evident that the long post outweighs anything that people might try to bring up. The thread dies out. Example.

Off-topic: If you read through the example for path 2, and maybe read some of the other threads by the same author, you can see the inspiration for <Crusher Of Dreams>. Hurr.

Howlitzers and Polyps.

I saw the thread that created Howlitzers from scratch myself. I was impressed when I found the implemented creature was almost exactly as the thread dictated, down to the concept art.

They read the suggestions, and lots get added to the game/changed.

I asked for the fountain in Town Square to be turned back on after Impostomas, and it was fixed with the next minor update.

If that's not progress, I don't know what is!

Fehzor is so lucky because his suggestion has been added into the game.