Character progression is more straightforward. Previously, when you leveled up, you were rewarded with a number of skill and attribute points that you needed to assign yourself, but in Diablo 3 your attributes raise automatically and skills are simply unlocked at specific levels. There's no cost for respeccing other than waiting out a short cooldown, and it can be easily done in the middle of a dungeon. It's great that you're never penalized for accidentally picking the wrong skill or required to put points into something to unlock what you really want, but each character also feels less unique as a whole. There's little reason to roll more than one of a single class as they can easily swap to any build.
What you've lost in the fine details of individually assigning points to attributes and skills has been made up with a big pool of skills, runes and passive traits. You can attach runes to skills for additional effects or even to change the skills drastically. For instance, attaching the Peaceful Repose rune to the monk's Serenity skill adds a healing effect, freeing up a different skill slot that I had been using to heal myself. There's a lot of fun to be had trying out different combinations of skills, runes and passive traits until you settle on something that suits your playstyle.
The biggest downside is that on the starting difficultly, none of this specialization or min-maxing really matters. You can easily plow your way through the game with little effort or thought to your skills. The only time my monk ever died was when the connection to the server hiccupped. Anyone craving a more challenging experience will have to complete the game, as that's the only way to increase the difficulty. On the opposite spectrum, though it's hard to imagine, if you found the game too hard at the outset, you're essentially up a creek without a paddle, except the creek is a river of fire in Hell and there are demons. If you are looking for an extra level of challenge, you can tackle Hardcore mode once you've gotten your character to level 10, in which death is permanent. Combined with the leaderboard, Hardcore mode harkens back to the days of old where surviving was the primary goal rather than outright defeating a game, and offers a fun side challenge to normal play.
One potential problem with playing Hardcore is Diablo 3's required internet connection. The need for an always-on connection to Battle.net ties in with the game's soon-to-be-implemented auction house that will allow players to sell items for real money in addition to in-game gold. These are welcome features, but even in the modern era of Wi-Fi, there are still times when you simply won't have internet access. Knowing that a small burp in your connection could kill your character permanently will sap the fun out of Hardcore mode for many a player. Multiplayer will doubtlessly be how many choose to play as well, and thankfully Battle.net makes it easier than ever to jump out of a single player experience, into your friends game and back again. So at least you're getting a little something in return for having to always be logged in.
Bottom line: Diablo 3 is a classic Blizzard sequel. The core gameplay has been iterated on and sharpened to a fine edge, while it ultimately feels a little light on major, impactful changes and the required constant internet connection will be a deal breaker for some.
Recommendation: The act of hack, slash and loot has been polished to a mirror shine in Diablo 3. Just don't expect much more than that.