This of course is dependent on the individual developer and what goals they want to accomplish. There are simple enough games that they could be created in a single day and there are those that take years (into mine over a year already!). Clearly the span of creation is a large factor for milestones. If you're not going much further than a mile, you don't pass that many stones :). That being said, there are probably some generic ones for the medium sized game.
(damn this auto numbering! I want to start at 0 like any good programmer would!)
Get an idea for a game. Easy, who doesn't have a list of those somewhere?
Choose your platform and language. You'll always see this question on this site. It's a big one for new developers.
Write the first line of code for your game. As simple as it sounds, this is a fairly big one. I bet there's 100 people who have an idea for a game they want to make but never start for each 1 person who completes this first milestone. They get stuck during a tutorial or they get so caught up in thinking about how to do it, they never do it.
Get something basic going. This could be as simple as getting a triangle to render on screen or using the keyboard to move a little sprite around. At this point you have a bare bones game. It starts, displays some graphics, responds to the player and then ends.
A million smaller milestones These range from getting your art assets loading correctly, to fixing that damn bug, to reading and writing to disk, to that premature optimization that was bugging you just too much to leave it.
A playable prototype. This should represent your finished product fairly well. Now things are starting to come together. You're 90% there right?
A million million smaller milestones. Oh the grind. You're deep in your second 90% with lots of little changes, fixes, re-optimizing your premature optimizations, brain meltdowns and code refactoring.
A complete game. Now you have 3 grey hairs and a finished product to show for it. Congratulations! Now you just have to publish, promote and maintain.
But really the milestones are a lot like this list. Made up on the spot and totally up to you.