It sounds like something has got corrupted with your save file - to the extent that Minecraft no longer recognizes it. Your best solution is to restore from backup.
I'm taking it you don't have a backup, so the next best thing is to try and open it with another program, e.g. MCEdit, with which you can make some trivial edits and re-save, which may help restore your save file to an acceptable state. Note that MCEdit is not tied to your Minecraft save folder, you can manually load a map from any level.dat.
If this doesn't work (MCEdit won't open the file), you will have to sort things out manually. You'll need to look at the contents of your save file, and see where things could be going wrong. A good reference is the Java class that creates the save files. Good luck!
The update notes you are looking for are not usually present on the tumblr, as you noted. There are two places, however, where you can find the information you seek.
Official Mojang blog
They can, however be found in the official Mojang blog on their main site. The older posts can be hard to find though, but should be able to find them with some googling, I suggest searching for
site:mojang.com -site:bugs.mojang.com minecraft [version]
to limit the results to actual results. Note that these posts are often updated, even changing the title in some cases, such as the one for 1.8.2.
Sadly, the information about bugfixes is often scattered among multiple posts for pre-releases as well as the main post, making them hard to follow. They also seem to be incomplete.
The wiki's version history
A much more convenient way of browsing the changes is offered by the wiki, in the Version History article, neatly sorted by major and minor version. There is an extra page for development versions as well, if you want to check out the snapshots or pre-releases.
The downside is that this information is user-curated and takes some time to be gathered, especially for non-obvious changes, such as the blacklisting of player head textures not hosted on a mojang server in 1.8.4.
Best Answer
The lag graph still exists (with a slight change to how it looks); you can see it by pressing ALT + F3.
The graph shows how long each frame took to render. The higher the bar, the longer that a frame took to render.
The numbers along the side should be taken as 1/number of a second that the frame took to render. If your data is around the 30 line, then frames are taking 1/30 seconds to render, and you will be able to get around 30fps.