The world will just generate with air blocks replacing non-vanilla blocks, and your non-vanilla items will just disappear. Re-installing the mod will not bring the blocks or items back, unless you do so before loading the world again.
(As a side note, some mods used to render your world unplayable unless the mod was installed again, I'm pretty sure Notch fixed it though, but there might be a few that mess things up.)
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
One way is to use item attributes. I've seen this approach on a few maps.
Using the command
You can give yourself a diamond sword with the maximum attack speed on it. This eliminates the cooldown of attacks.
(PS: you can find more about attributes on the wiki, here)