The Legacy of the Void developers were making a bit of a song and dance about patch 3.0 for all versions of StarCraft, and they weren't kidding -- it's bloody huge.
You can read the patch notes in the Battle.net launcher, but one user has neatly summarised just how many changes there are by breaking it up into an Imgur gallery.
There's over 6 pages of adjustments, with the biggest two changes being the opening up of the Whispers of Oblivion campaign missions to all players (it was only previously available to those who pre-ordered Legacy of the Void) and a total overhaul of the game's UI.
Custom games can now be found under the Multiplayer tab, Archon Mode has been made available for Blizzard sanctioned maps, in-game chat now all takes place within a single persistent window, and the Arcade page has been altered to revolve around the "Join" and "Browse" tabs.
It's easier to parse the changes through images, so here's a walkthrough video from Blizzard that easily outlines it all.
I do like the little nod to the campaign selection screen from Brood War -- although it's actually appropriate with StarCraft 2 having three separate single-player campaigns (the original and its expansion, Brood War, had campaigns for each of the game's races, whereas the story for each of the races in StarCraft 2 has been split across the release of the vanilla game and expansions).
Other major non-UI additions with Patch 3.0, which is available now for all versions, include the removal of matchmaking for free-for-all matches, a 64-bit launcher of the game and the inclusion of "alt+number" control groups.
If you're familiar with the SC2 editor, it's also worth noting that it now has the ability to merge maps and mods. A tonne of options have been moved or shifted around too, so you should probably study up on that if editing is your thing.