For the changelogs you can check out any of these (listed in order from most to least recommended, if you're only going to read one):
1)
Wiki Release information - includes links for each release to lists of bugfixes, tweaks, and new features given by Toady One and his comments from the dev log and the forums. It's concise and fairly complete.
2)
Development log - includes lists of bugfixes, tweaks, and new features, along with Toady One's comments on where the game is going. It's not concise, but complete and adds other interesting info.
3)
DF Announcements forum - includes lists of bugfixes, tweaks, and new features, along with some other comments from Toady One. It's concise if you only read the first post, but long if you don't, though there's other good info hidden in each thread.
4)
Bugtracker changelog - includes bug description links, but does not list new features. Concise and informative, but incomplete.
5) My ultra-concise but leaves a lot out list - many bugfixes, some new bugs, some basic improvements, animal domestication redone/improved, wheelbarrows and minecarts added, many hauling improvements, item weight now affects hauling speed (use minecarts/wheelbarrows for heavy items), falling damage changed (falls less hazardous, while items dropped on creatures became more dangerous), workshops can be linked to stockpiles, and stones and ores now drop 25% of the time (instead of being based on miner skill) but stones and ores now produce x4 blocks and bars, respectively, to make up for that (bituminous coal and lignite also produce more coke). EDIT: Oh yeah, and adult dwarves now require new(ish) upper- and lower-body clothing plus footwear or they start getting bad thoughts, so don't neglect your clothing industry. (Thanks, King Mir.)