Aliens need more power.

[Howitzer]

I can live with the fact that aliens have a simple 1x dmg 'multiplier' over there whole body.
This is also a bonus for humans, ex:
Dragoons mostly exist out of legs, if a human shoots the dragoon's legs, he won't do less damage, which is good, because he will mostly shoot it's legs.

[Workaphobia]

But would alien heads be as vulnerable as human heads? They're aliens after all.

If you shoot us, do we not bleed? If you fire grenades at us, are we not fragged? If you...

My vote goes for stronger alien defensive structures such as, gasp, ranged defenses that stand a chance against pulse fire from a corridor. An actual use for barricades would also be nice, or if that's not going to happen, at least a cost reduction. Really, who wants to spend 10 sentience on a small useless pillar that can be destroyed in three seconds, when you could be hiding a useful egg somewhere?

Also, dretch is a very useful class in the early game, but there should be a way for aliens to effectively attack bases besides saving up for mass dragoons or tyrants. Would it be totally out of the question to add an additional starting unit - something that sacrifices stealth for a bit of an armor boost against turrets? Humans have all sorts of possible armor and weapon combinations, but alien evolution is a pretty linear path. Why not add some branches and a new root?

We have our zergling and our ultralisk; now give us a lurker or guardian.

[Silverius]

If your defense structures can be shot from a distance you've placed them badly.

And there is a way for aliens to fairly cheaply attack bases: mauraders. Two kills gets you one and they can deal a fair bit of damage to a human base.

As for barricades, they indeed don't seem to have too much use, but then, at s2 you can build trappers which are effective at stopping the opponent. So if you're wise you only use barricades at s1, the stage in which they actually are somewhat useful.

[b0rsuk]

Marauders are good at two things:
A) jumping inside of unguarded human base,
B) drawing a lot of attention from the enemy while spamming "Enemy is distracted, attack !" hotkey. Distractions is where marauder really shines. Jump like an idiot on the end of the corridor, and marines will soon be forced to go back to base and restock ammo. You can easily hold 3 not-too-good marines with one marauder. If they got so brave they're going outside their base, your overmind is probably already under attack or something. Preventing 3 other marines from  either attacking overmind/buildings or defending base for little or no cost is good.

They're not terribly good at killing, but can kill. After checking marauder zap closely, it indeed deals 80 damage if you stay in range. Good for enemies easy to hit, ambushing from behind, or guaranteed hits against groups. (groups of buildings, too). 40 damage claw is nice, but not spectacular considering dretches have 48.
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Think about human base defences as turrets, and alien base defences as traps.
Hives can be nice, but place them where enemy doesn't care to look. He should be distracted with something else. Hive has only 125 hp.
Acid tubes - it's rarely worth it to put more than two in the same place. They get really useful on stage2, when you can place them on walls. Force marines to come out of doorway to shoot them. Then eat them.
Even one acid tube is good between you and enemy, because it will slow a battlesuit for at least several seconds, empty his ammo and of course do damage. Humans have limited ammo and only one medikit, so gradually bleeding them works well.
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Barricades....
USE HOVEL INSTEAD OF BARRICADES.
Hovel is free. It costs only your time.
Hovel is much more durable - 375 health (barricade 200)
Hovel regenerates very fast - 20 (barricade 14)
Hovel is much bigger than your eyes tell you.

Just don't place it anywhere near tyrants.

[Greewi]

An other realy good technic is the egg hidding... With 2 granger that go everywere in the map a place eggs anywhere it can suvive 20 seconds, is sufficient... And for the overming... It build itself quite quicly... And the time that it can suviver, th other granger can build other eggs... As there is no defence, there will be many eggs !

This technic is relativly sufficient to live to stage 3... And you know what mean stage 3...

Alien are the master of guerilla ! Dont forget this : be everywere, the best is in the back of your ennemies !

[Silverius]

However it can be useful too to build a bit decent base and defend it and simultaneously have some others hide eggs.

And folks, when you're under attack, start as granger, not dretch. You can just as well evolve from a granger as a dretch and when egg hunts are going you need to constantly hide new eggs / om.

Yesterday I played a game where in a team of five I was the only granger. Worse even, the rest didn't respond to my request to protect the overmind (which was in an at least midly defendable place). We did make it to s3, but because people got out solo and got themselves shot we didn't get tyrants and lost all the eggs.