Heroes of Newerth:

(That's an older youtube vid I found. I'm not allowed to publish new material from the beta.)

A beta-leak video review of HoN (German). Shows you much of the lobby interface and other features.

League of Legends:

An official trailer for LoL.

A video review of LoL (english). Not sure if leaked or sanctioned.

DotA (Defense of the Ancients)

Some DotA video. I honestly couldn't find ANY good gameplay video for Dota. Most vids are simply my-dick-is-BIG demonstrations accompanied by some shitty trash metal soundtrack or something bombastic (like, US-marine corp recruitment orchestrals or Carmina Burana). Sorry. If you can recommend a good DotA video I'll put it up.

Demigod:

Demigod showcase video featuring Lord Erebus.

Demigod showcase video featuring Queen of Thorns. Probably not office safe.

For those that don't know what HoN or LoL are: Both are DotA clones. If you don't know what DotA is then google it.

First HoN impressions:

• Graphics
  Looks like Warcraft 3 (i.e. old, cheap, candy colored comic look). Seems much smoother, though. Less jerky scrolling. Any comparison to Demigod would be unfair. It's like comparing Superman to Batman - both are great in their environment but if you'd match them up against each other Superman would wipe the floor with Batmans ass from two cities away.
  Has a limited zoom (i.e. you can zoom in a bit and back, but not out) and a functional minimap.
• Interface
  Just like DotA, i'd say. Bottom quarter of the screen is occupied by artwork, skills, hero stats and the minimap. I'd call it functional but totally ripped of from Warcraft (which isn't necessarily bad). Very small icons and text. Hard to read and the tooltips have much text. Demigod is downright minimalistic in comparison.
• Heroes
  A large selection of heroes ... too many to grasp the gist of them in the short time I played it. Not all heroes were implemented yet but it looks like they planned for a 60 heroes cast. Each hero seems to have four different skills you can improve as you gain levels, plus the ability to simply improve your attributes. The skills are either passive or active and have a cooldown timer similar to DG. But since some heroes can have several passive skills out of a set of four, that can mean that you are left with a meager single one active skill, like the tosser I picked (Magebane or something). Kinda like Oooze UB without foul grasp.
  However the skills some heroes have do things that are not seen in Demigod. Some manipulate the map (by planting obstacles(walls) and some move the enemy against their will. Some passive skills trigger a damage effect everytime you use an active skill.
• Maps
  Think Warcraft again. Mazes with trees and rocks marking the boundary. Or think Demigod maps with boring graphics. They ain't really that different from the DG mazes ... just more "realistic" and much larger.
• Players
  Lot's of open games in the lobby. Some fairly large ones, too (I think 5v5 is max). If you've played DotA before then you'll have no problems finding friends. If you only played Demigod before, then you'll think the HoN community is a bag of arseholes because you'll likely be taking your first stumbling steps with dota veterant that are not exactly renowned for their forgiving attitude towards noobs. Luckily there's a fuck-filter in the chat.
• Economy
  Killing stuff earns you XP and gold. You can buy equipment in shops. No base building, but your base starts with several buildings that don't offer any interaction, though. Creeps spawn in certain buildings and run on lanes to the enemy base, which should sound familiar.
• Death & Dying
  You get automatically resurrected after some time. You can, however, shorten the wait time by paying a moderate amount of gold, which is actually an interesting feature. Similar to DG, there's a Healing Crystal equivalent, something that had to be pointed out to me in the Demigod forum (thanks, dude).
  
• Gameplay
  Weeeeeeeeell .... I sucked in my first game and got promptly verbaly punished for being a noob. This doesn't help endear you to the game, so my perceptions may be flawed. But my first impression was: I can't zoom out (I swear, after playing Demigod I think it's impossible to enjoy RTS games without a God-Eye-Zoom)!
  In addition to the fixed camera height you have a considerably larger map and a faster movement speed than experienced in Demigod and you can't help feeling confused and disoriented. The creeps are stronger in comparison to DG creeps (but then those puny heroes are no half-gods). There is no health crystal. I think the only way to heal up is to buy potions. At least you can buy more than three per slot.
  The game is fast paced but not so fast that you'll experience instant death right from the start. You'll have plenty of time to decide that a battle is a lost cause and run for the towers.
  And your job? Easy, kill and destroy. No flags, no warscore (although there are other gamemodes that might have something similar). The only thing you can do is find creeps or heroes and kill them. And find enemy buildings and destroy them. That's it.
• Game Modes
  There are only custom games. No skirmish or pantheon. No single player mode and (currently) no bots to solo against. Many possible options to setup a custom game. And the features the lobby offers are something to be seen. Feels like a database browser (i.e. comfortable and powerful).
  
• Replays
  They work. Take that, Demigod!
• Community
  It's a new game so you'd expect the community to be new also. But it's not. It's mostly DotA veterans that got a bit frustrated with the aged Warcraft 3 engine and wanted something, er, newish. Someone in the HoN forums summed it up pretty much: "I'm actually pretty surprised by this community. So far it's been very hard to find nice people. Most players - particularly those with Dota experience are incredibly rude to all players."
  The Demigod community certainly has it's share of unfriendlies, but as a whole it's quite amiable. Maybe this will change with the rise of competetive play.

Bottom line: Graphically it's definetly a step down when compared to Demigod. But noticably better than DotA. It's really really running smooooooothly. If you consider the small DG roster Demigod's biggest drawback, then HoN might seem attractive. The complexity and detail of each hero is lower than that of any one Demigod. It's also extremely beginner unfriendly. And even though Demigod didn't have any kind of tutorial either, you grasp the gist of DG way faster than that of HoN. HoN is clearly aimed at DotA players. It looks so similar to DotA that fanboys might as well stay there.

It's still in beta so many things will change. The strongest point of HoN is it's large hero rooster. Apart from that I don't think the Demigod has much to fear.

But since it's an early test phase, I'll certainly give it another try. I can endure the "F*CKING _NOOB" for a time, hehehehe.