To me, its like the difference between starcraft and nearly every othe RTS I've played including C&C, SupCom, DOW, Dune (way back when), Sins, etc.
In starcraft, every race is very unique. Sure you may say the races have unique things about them in the other RTS, but the unit functions the same in the same category across the board. There are slight differences (maybe one race has more hit points, maybe one has more damage, maybe 1 is faster), but they serve the same purpose and are countered about the same way. It makes balancing the game much easier. Race 1 has unit A, B, and C. Well lets makes sure Race 2 has A, B, and C and we'll just tweak the numbers and models to make them different. That way Race 1 and Race 2 are different, but have nearly equal options available and thus are balanced.
This is not the case with starcraft. Sure, there are units available to each race at different tiers, but they are very unique from each other. A zealot compared to a zergling compared to a marine. Range, cost, health, damage, are all very different but they are all fairly balanced. This means you play each race very differently. Each race has different tactics and overall strategies.
The point is, from what we've seen of beta 1 (which we all know is just a glimpse), Demigod is more like a C&C, SupCom, or Sins then a starcraft. There is some uniqueness in each demigod, but they're all pretty vanilla. They all have a stun skill, they all have a high damage skill (nuke), they all have an AOE nuke, they all move about the same speed, and really all play pretty similarly.
Yes, I know they have some differential such as the Rooks ability to put down a tower, or eat buildings. Those are great examples of distinguishing him. Also, the torch bearer has his fire mode which is a nice way to make him unique. Regulus has good passives (range, cripple) and mines, but theres not enough differential between each of the heroes. I'm not talking about balance which will tweak the numbers (cooldown, duration, damage, cost, etc), but about the actual skills.
Dota is more like a starcraft. Sure, with so many heroes some start to play very similar (see lion vs Luna or Slaardar vs Centaur), but there is pretty good distinction of play style between the different types (ranged int, ranged dex, melee str, melee dex, melee int). Throw in a few very unique skills and the game has many flavors to choose from.
This is all based on Beta 1. Generals are still to be added, more maps, more content, more tweaks will surely come. But, just looking at the 3 available demi's, models and animations aside, they are too similar at this point. Just my opinion.