Having mucked about in the beta for only a few hours today, I can list off a short list of changes so far.
Chat interface is greatly refined and far smoother than before.
You get Rewards for 100% exploring an area, including loot; having fully-explored Divinity's Reach in the first BWE, it was a pleasant surprise to revisit the City and see a chest icon in the bottom-right that gave me 1K exp. Doing the same in the first explorable area gave me a weapon, some armour, a pile of transmutation stones, and a heap of crafting materials, as well as Experience, karma, and money.
Melee combat has noticeably changed; 'normal' attacks from enemies of the same or slightly lower level than you only really do 'chip' damage; Having my level 20 thief downscaled to 12 in beetletun, the local centaur population could only really hurt me when they fired off a special, like their warrior's blindingly obvious charge attack. My level 10 warrior also chumped the same centaurs by merit of having a big Fucking hammer to knock them flying.
All melee weapons have a noticeable physical buff; my old and crappy daggers were doing 20+ on baseline hits instead of 10+ in the previous BWE, pistols and rifles remain the same, but have what appears to be a slightly slower Rate of Fire, balancing the advantage of 'keep-away' with lower DPS.
The doubling of servers (96 worlds in the beta = holy crap) means that the only lag is client-side, not server-side. Trawling the forums basically showcased that everyone who complained about lag in the last BWE has mysteriously disappeared entirely.
They're reworking the minimap; last BWE it was a classic circular minimap akin to the GW1 'radar' map, this time it's rectangular, and clearly a placeholder.
Utility skills have tiers to encourage experimentation; you need to unlock 5 skills per tier to gain access to the next, and there's 7-9 skills per tier, not counting racials. Tier 1 skills all cost 1 skill point, and have fun things like the thief's scorpion wire, or the ranger's summonable spirits that buff allies in fights. Tier 2 skills cost 3 skill points each, (so at minimum, you'll have blown 20 skill points to gain access to tier 3), and, have more of the 'generalist' utility skills, or AoE skills, such as the Thief's blinding powder (Blinds all nearby enemies, stealths all nearby allies). In general, I find that it encourages experimentation rather than settling into one build, which is a GOOD thing.
Traits are also tiered; instead of getting access to all 12 major trait choices, it encourages investment; trait 1-6 are available at tier 1, 7-10 are added at tier 2, and 11-12 are added for tier 3, so you get a limited selection early on, but the more you invest and specialize, the more powerful major traits are unlocked.
Remember; all of this is still subject to change, for obvious reason.