The most basic stuff – the fundamental core d20 system – is largely the same. That is, you still roll a d20 and add a bonus, and check against a number. Bonuses of the same type don’t stack.
Various character options (races, classes, feats, class features, spells, items) are all new, even if they have the same name as a 3.5 option. I’m not going to go through each of those, since you should read each during character creation anyway.
Philosophical Differences
For a 3.5 player, though, I think the primary difference to be aware of is a matter of philosophy. Rule of Cool had exactly one primary goal:
To give you rules that, as much as possible, facilitate play, and otherwise stay out of the way.
(paraphrase, not a direct quote despite the box)
Options are designed to be streamlined and fast to play. A lot of things that were dropped from 3.5 were dropped because Rule of Cool decided that the rule took more time – to learn, to look up, to judge – than they were worth.
An excellent example is flight: instead of 3D flight, which was frequently a source of headaches for many tables, Legend just uses a special status condition, [Flying], for creatures in flight. Rule of Cool felt that making it fast to run was more important than creating a flight simulator.
This is very important, I think, for a 3.5 player to understand: Dungeons & Dragons is weirdly bipolar about simulation vs. abstraction, being very abstract in some cases and trying very hard to “match reality” in others. Legend is very much more focused on being a game first. This is obviously not for every player, so it’s important to know it going in, and for those giving Legend a try, I think you need to come with those expectations.
Gotchas
Anyway, some specific gotchas:
Natural-1s are no longer auto-failure, and natural-20s are no longer auto-success.
All Races get a bonus feat at 1st level, not just Humans. This is in addition to their normal 1st-level feat. The feat can be any one of the feats listed in their Race entry, any [Racial] feat that names the race or lists the race as a prerequisite, or the Guild Initiation feat. For the feats listed in the Race entry, you may ignore the feat’s level requirement (but not other prerequisites). Races with a Bonus Feat entry of “All” (such as Humans) may not ignore any prerequisites.
Ability scores are no longer intrinsically tied to combat stats. Your HP and AC depend on your class’s Key Defensive Modifier; your Attack Bonus and Damage rely on your class’s Key Offensive Modifier.
Gotcha even for players without 3.5 experience: physical melee attacks always add ½Strength modifier to their damage, even if Strength is not your KOM. You always subtract ½Constitution modifier from physical damage taken, even if Con is not your KDM. Easy to miss those two, and they’re kind of an exception to the previous point; these were necessary to balance out the fact that Strength only covers a couple of skills and Constitution only has one.
Saving throws work like 4e: Fortitude adds the higher of Strength and Constitution, Reflex adds the higher of Dexterity and Intelligence, and Will adds the higher of Wisdom and Charisma.
The base saving throw bonuses from your class use different progressions than in 3.5.
There are no ability checks. Any situation where you would use an ability check in Dungeons & Dragons should be a skill check in Legend. The closest applicable skill, or any one of a group of applicable skills, should be chosen.
There are no skill points; you either are trained (and add your level to the check) or you’re not (and don’t). This is a matter of streamlining.
There are no opposed rolls. Instead, every character has “defenses” against such skills. For example, instead of Hide vs. Spot, it’s a Stealth check against the observer’s defense against Stealth, which is 10 + level + Wisdom modifier. This means that hiding from anyone of the same level is a challenge for a stealthy character, but a stealthy character should have a good chance of succeeding due to focusing on Dexterity more than most will focus on Wisdom, and possibly having other bonuses to Stealth.
Combat maneuvers are similar: most force a Fortitude or Reflex save against 10 + ½ your level + your Str or Dex modifier. Even grappling is merely three maneuvers (Grapple, Pin, and Choke), that force saves against some status conditions (namely [Grappled] and [Pinned]).
Critical hits no longer need to be confirmed, but add only twice your level in damage.
Weapons no longer have different base stats: they all have a base of 1d6 damage, either [Melee] or [Close] range, and crit on a roll of 20. Physical damage types are not differentiated against. Weapons also get “Weapon Properties” that separate them – these are things like Reach, Brutal (for larger damage weapons), Tripping, etc.
Legend also recognizes non-combat encounters, and uses “Skill Games” to judge them. This is how Social Encounters are handled, as well as things like races through difficult terrain (the Parkour skill game) and a trying to riddle out a mystery (the Investigation skill game). The idea is that you have more options than simply rolling your skill and hoping to hit the DC; instead, there are special action types that you can take during these types of encounters, and you use your skills to decide if those actions succeed and improve your standing in the overall encounter. Different actions can give you different advantages, just like in combat.
I’m sure I’ve missed things, unfortunately. I will try to expand this answer over time.