Lots of questions.
1) Are you imposing rules on what kind of achievements a game can include? If so, why and what?
2) Will our games be tied to some system on your site like steam or live, requiring a player to sign up just to be able to play it, or can they continue to download it from anywhere, play it, and if they choose they can later sign up for an account and play through again to get achievements?
Games can be downloaded anywhere, played anywhere, etc. They will not be able to save trophies without an account, though. High scores will most likely have two options that a dev can choose to use for their game: either guests and users can submit high scores, or just users. All up to the game developer how they want to handle things.
3) Further to the above question. What do you have in place to handle issues caused by someone either playing the game offline, playing the game without creating an account to hold achievements, or intentionally wishing to avoid signing up for whatever reason.
Up to the game dev. If they want to store the achievements in a file, they can.
4) Is this a free service, or bloated with ads or are you charging game makers?
The site has ads. Not many, though. Games don't have to have any ads pointing to Game Jolt, although of course it'd be appreciated. Without ads on the site, though, we couldn't do this:
5) What does the process involve? For example, a game maker would sign up, then create a "game account" add their achievements, and then somehow link up the plugin to those?
Yeah, pretty much. You can experiment if you want. Games are by default hidden from view when you add one.
6) Are the achievements shown on the site following a uniform appearance (like Steam and Live do with theirs), or are you allowing games to provide their own styles. By this I mean I know the actual icon (using the above mentioned examples) are always specific to the game and achievement, but do you still have some guidelines that need to be followed, if so what are they?
The look and feel on the site is unified. How it's portrayed in the game is up to the dev. For the site, devs can upload a 100x100 thumbnail of the trophy, though. They can also add a title, description, and difficulty setting (bronze, silver, gold, plat.).
7) Will this service result in forced branding on games?
No. Game Jolt has always operated as a service to help the game devs and not to force crap onto them.
8) How deeply does your achievement system impact a game? Does it sit quietly in the background within the game, accepting input only when required and uploading it to the site. Or will it need to be added to the game at the ground level and integrated with everything from the start?
Not sure what you mean. You tell it to do the commands, and then it does it. You could easily create a whole game and then just add in some trophies. It's pretty modular.
9) If your plugin requires very deep inclusion with the game, how well does it play with possible future achievement plugins from elsewhere? Some game makers may wish to link up to a number of sites for whatever reason (including their own perhaps), how will yours effect this or wont it cause any problems?
Don't see any problems here.
10) Is your plugin tied to Game Jolt, or can it be used, like with the above example, for the creators personal site too, for example if the creator decides to have a page displaying the overall number of players who got each achievement on their own site (without requiring them to have anyone sign up, just have it updated automatically), while also using the plugin to update more specific player accounts on Game Jolt (as well as I imagine an overall page there too).
There is an alpha Game Jolt Service API, which nearly any piece of public information can be pulled from the site. Devs can use this to pull in trophies or leaderboards or anything into their site.
11) Are there any limits to the achievement plugin, or does it accept variables (and thus practically anything you wish to throw at it)?
Right now it only supports trophies. Very soon it will support data storage, in which case you will be able to store anything. It'll most likely have to be serialized in some form as a string or something. This can be used for level sharing, replay storing, save state, etc. Really anything.
12) Are you insisting on a specific "achievement unlocked" icon/image to be used in game, if so why, what, and what's it look like? Otherwise I take it we can have our achievement unlocked icons/borders etc. look however we want (in game)? But will you also be providing a selection of usable image files, for users who either can't or wont create their own, to at least provide a base level of quality?
You can use whatever. We will be releasing images to supplement the system soon, though. The basic trophy icons used on the site and stuff.
Hope this answers your questions.