Detailed first impressions

We'll take this part by part and go over them in detail.

Overall Gameplay
The ebb and flow of the game is fairly dynamic with many influencing factors: heros, hero abilities, units, unit abilities, towers, deployable neutral turrets or cubiods, netural enemies protecting bases, upgrades of all of these things, etc.

You definitely see different stages of the game.
Early game is grabbing quick expansions and collecting gems while trying to deny the enemy collecting gems. It's relatively easy to pick off heroes that over-extend early game too based on the amount of damage you have available immediately and the generally low health of heroes.

Mid game looks like it's very tower centered, setting them up busting them down, pushing along a "lane" a ways and then backing off to hit another. This feels good but chaotic because the map is very large and there's a lot of content in them.

Late game is crushing defenses with overwhelming force while setting up neutral turrets and such to deny putting them back up. You see much larger and powerful surges of attacks, I started to see some actual deathballs of 2-3 heroes and their late game units terrorizing the center of the map.

Reinforcing units is such a good concept. I love that you pay once and then they respawn when they die. I dislike how little units you can actually make in the end, although if you allowed more then their might be considerable issues with visuals, performance and general chaos.

Due to the limit of how many units you can build you tend to naturally lead into a mass-upgrade phase. Although I like to get damage and attack speed upgrades before T2.

I totally expect when the game comes out for the "best" build orders to develop very quickly. It would only be a little bit of math to see if it's better to get +10% attack or +8% attack speed at any given time and # + type of units. I think that's just the nature of how upgrades are handled in the game though.

** Upgrades **
There are a lot of the general +% or +X boring upgrades that aren't interesting at all. Skill trees or upgrades that just increase what you do already are necessary for sure but they just end up being a math problem rather than an interesting decision to make. This would be a design decision and I'm not entirely sure what you could instead.

** Performance **
I haven't had any issues with the game, the few times I've looked at the FPS it was well over 130+. I'm running an i5 2500k (sandy bridge), geforce 970, 32GB of ram (yes that's a lot!), and the game is installed on an SSD. So this is running as expected (perfectly) with everything maxed even in a pre-alpha unoptimized state.

Loading screen / Squad selection
You can see in chat what the opponents select but the avatar for their hero doesn't show up, only the allies. I can't tell which is supposed to be correct, but it's my guess just a visual glitch and not a game design issue. In-line with that the game freezes briefly when it goes from loading screen->in-game and flashes some weird graphics.

Pre-Game
I'm unsure if I really like building units/upgrading before the game starts. If the idea is to have a grace period for people to get ready then it still punishes those that don't build immediately because others will deploy before if you are slow enough. And why do we have to deploy them out when the game starts? Why would we NOT deploy them? Really just end up mashing V in the final seconds, which I guess can be mechanically fun but not really interesting.

Map Design / Flow
I think I'm seeing a trend of the very top left of the map seeing a lot less action than the centre and bottom right, despite the 2v2 being closer to there. It's like that top left heavily defended neutral base doesn't exist since it's so risky to take it. On contrarily I've seen several times the far bottom right be under heavy fire constantly.

Hiding in the tall grass/bushes/LOS blockers is hard to know if the enemy can actually see you or not, sometimes they can and I have no idea why.

The cause-effect relationship between shooting and dealing damage feels really weak (for Vela at least). All you see is the health bar go down but I think the main issue is missing sound effects or visual effects to go along with. It can be difficult to know if units are actually dealing damage or if they are canceling out their attacks when stutter stepping/moving and shooting.

** Gems **
It's both interesting and odd to have them just spawn randomly. It makes it more dynamic in that you can't camp spawn locations at specific times but it also means you are at the whim sometimes of getting 3 easily defended close together. Gems appear to drop with more value/larger when you kill something of greater worth as well, which is cool and rewards not just killing something but staying around to collect it. Means board control is that much more important and not just hit his hero for xp and run.

** Units in general **
I still have no idea how to know who or what I have selected. They have circles around their base but that appears to be it. I haven't seen any indication in the hud who I have actually selected.

** Hotkeys **
So you can hotkey building and units but if you hotkey the upgrade building and upgrade something, you lose your hotkey? Kind of annoying to have to go back to base to click on them to start my next upgrade.

** AI / Pathfinding / Etc**
The AI is pretty awful I feel. Pathfinding is hideously bad at times with units running back and forth forever. When units collide they don't push each other out of the way or go around, they just deadlock. If you as the hero build a neutral turret you can trap yourself and have to recall with no way to cancel the building (that I know of). The AI doesn't work together overly well and is often seen running off on its own deep in enemy territory with very clearly enemies on 2/4 sides of it and only a few units. At other times it runs way seemingly for no reason despite having a massive advantage or I am coming to support.
Collision detection in general is pretty frustrating because the hit boxes are all much larger than the units themselves. Combined with the bad pathfinding it results in units being totally useless. Having to move snipers in close so the others can actually do something is counter-intuitive.

... continued in next post because it's too long