Not XP spent on class features, more like on power-suites. Build strength is established via an XP-earned total, much like a BP pool, but the classes provide a bit more structure than a fully-open freeform system like Mutants and Masterminds (or Warhammer, as you say, though I've not seen it), and provide a capability-by-level guideline to follow. It's also intended to follow a growth curve like regular D&D, starting low and building into major power.

My general reference is using the more structured flow of powers and level progression of 4e, but with more of the mechanical flexibility of 3.5.

  • All six stats have use on any character, intentionally. Strength, Agility, Endurance, Intellect, Vigilance, Charisma.
  • There are four 'saves', Brawn (strength), Dodge (agility), Spot (vigilance), and Will (charisma), though I need a new name other than 'save'.
  • Powers (spells, maneuvers, tricks, and other things) are unified into a rank scheme akin to 3.5 spell/maneuver levels (including 0-level), 1 per 3 levels, which can be purchased through skill points.
  • Skills follow the same rank scheme and level basis, but are entirely used in out-of-combat challenges (in-combat things that would be handled by skills typically use the four 'saves'). So, like, Arcana, Leadership, Streetwise, History, but not Jump (Brawn check), Listen (Spot check, which works for all senses), or Tumble (Dodge, or 4e-style Shift speeds).
  • Characters start with their Endurance score in hit points at 0-level, and get 5+End-mod hit points every character level, with extra HP granted by whichever classes of that level they possess. (Fort saves for 'death' effects generally don't exist, more things are straight up damage points. Poison is a damage type, like 4e.)
  • Character also start with their Intellect score in skill points, which can be spent on whatever. (this corresponds with the 4x first level from 3.5, but because both skills and powers use these, you can put your focus anywhere you like. Basics will be covered by your class's granted powers, so these allow extra things to be tried, variety to be included. Class-granted powers also come to you a level earlier than you can just buy a given rank with skill points, driving home the point that the classes are the best way to do a thing.)
  • Class levels grant 'surges' per level, which function like a hybrid of healing surges and action points (3e and 4e), though these other uses are typically accessed through powers.
  • Powers generally favor an encounter or at-will use, of a sort. Most powers have a cost, but that cost is reducing or negatable in particular circumstances.
  • Most spells use Mana, which is directly matched to one's Charisma score. Classes based on a particular type of spell make that spell cheaper. Powerful spells (4e Daily-like) use larger starting values. The fixed pool size forces the character to 'focus' to regain their mana when they've run out, and makes an experienced Summoner who picks up Evoking not instantly rolling in excess mana.
  • Maneuvers functionally use a ToB-type system, with all three refreshes at once, for most manuevers (though some are 'at-will', and are excepted from this). At the outset, they use the Surges mentioned (but you don't really want to do that, waste of good resources unless in an emergency), but there are three 'refresh' methods that negate that cost, making it free. As a costly action (such as a move+standard or something), you can make several (based on your Vigilance modifier) maneuvers free, as you spot the environment and see several openings. As a lesser action (like just a move or standard or something), you can get one maneuver of your choice for free, as you figure how to make that one special trick-move work. And finally, if you don't do either of those, you can get one randomly-selected manuever refreshed for free at the end of your turn, as the battle shifts and you spot a new opportunity.
  • Others power types use different methods. Alchemy and tech devices are 'fire and forget' in that you prepare them, typically, during your short or long rests, and then expend them as needed. I'm trying to keep them fairly concrete, rather than over-abstracting.
  • Opportunity attacks don't break turn order. Instead, when a character crosses your threatened space, you can choose to automatically deal a flat/simple amount of damage (which their armor may ignore) or save the opportunity, allowing you, on your next turn, to attack as if they were in melee. Since the 'round' is an abstraction, your turn includes all the actions that occurred since the end of your last turn. On the other hand, that attack is still just your main attack action.