Character creation...
  • Attributes are approximately the same, though point-buy is more strongly encouraged and the costs for different attributes are different.
  • Races are changed quite a bit. Every race (except humans) get +2 to one specific attribute and +2 to one of 2 other attributes, +2 to two skills, usually 1-2 other minor perks, and a single racial power that is almost always an encounter power (that is, it recharges after taking a short 5-minute rest). There's no such thing as a racial attribute penalty, so any race can get an 18 in any stat (though they can get 20s in some).
  • Hit points are never rolled; a character of a given class gets a particular number of hit points at level 1 and per level, with the only variance between different characters of the same class being due to their Con score.
  • Skills are either trained or not, and there are far fewer skills than in 3.5. There are no skill ranks. Basically, 2-5 skills from 3.5 were all rolled up into 1 skill in 4e. You get a +5 bonus (quite significant) to skill checks you're trained for and can use the skill for a couple things that you wouldn't otherwise be able to do, but the designers wanted to specifically avoid circumstances where a task that was reasonably challenging for one character could be completely impossible for another, so that just basically doesn't happen due to the way skills work.
  • Feats are given more often (1 at L1, with another at every even-numbered level, L11, and L21), but are far less powerful and character-defining than in 3.5. A feat never gives new options, instead simply enhancing those you already have.
  • What does define characters is mostly the powers they choose, which are most similar to 3.5 spells or ToB maneuvers. There are 4 main categories of power: at-will powers that can be used constantly, encounter powers that recharge after a 5-minute "short rest" which will normally be done after each battle, daily powers that recharge only after an 8-hour "extended rest," and utility powers that can be any of the previous and that have effects that don't directly affect enemies.


Combat...
  • Generally, the combat system is far more unified than it is in 3.5. That is, everybody's working off of the same rules; you don't have one person using spells, another using maneuvers, a third using psionics, a fourth with normal attacks, and what have you.
  • Everybody gets 3 actions per round, one a standard action generally used to attack, one a move action usually used to move around, and one a minor action to do things like draw weapons or use utility powers. This means that it's usually not possible to attack more than once per round.
  • When a character tries to do something to an enemy, it's the character who rolls to see if they succeed. That is, instead of a wizard casting Sleep on the enemy goblins and the goblins rolling Will saves to resist, the wizard rolls an attack against the goblins' Will defense. Mathematically it's identical, but it's another part of the system unification: the attacker is the one who rolls attacks.
  • Practically everything that you can do to enemies will also damage the enemies. That is, if a cleric turns undead and hits with the attack, not only will the undead be forced away, they'll also take significant amounts of damage. Exceptions tend to be powers with crazy-strong effects that are actually encounter-winners all on their own.


I'm sure there's more, but I keep forgetting what I've already written when trying to figure out what actually needs to be said The system works well and you'll probably enjoy it, though it is quite different from 3.5.