Venomancer is currently being picked as a semi-carry or a greedy support hero. With powerful teamfight and push/anti-push, the hero can be a great asset to a team... if you can find somewhere to lane him.
My own experience with Veno is that without farm priority, the hero is extremely slow and squishy, similar to Crystal Maiden (CM), but without the early-game power that CM gives. This basically means that Veno ISN'T a support, and other than trying to get a very-early kill with, for example, Venomous Gale + Blade Flurry, the hero should be played as a snowballing semi-carry.
Nerfs to Venomous Gale:
It should be noted that Venomous Gale's early damage, and Veno in general, has been severely nerfed in recent patches. Check this out:
6.85: Damage = 75/250/425/600 (or 56/187/319/450 after typical magic resistance)
6.80: Reduced base attack damage from 24-26 to 19-21 (total attack damage is now 41-43), and reduced base health regeneration from 0.75 to 0.25
6.79: Rescaled damage over time from 15/30/45/60 to 0/30/60/90 (total damage from 250/500/750/1000 to 25/500/975/1450)
6.76: Reduced initial damage from 50/70/70/100 to 25/50/75/100, and increased damage over time from 10/20/40/50 to 15/30/45/60 (total damage from (200/370/670/850 to 250/500/750/1000)
What do these changes mean?
6.76 - Veno's Gale initial damage decreases, but total dmg increased. Target can be denied more easily.
6.79 - L1 Gale damage drops from 250 to 25. Veno a lot less relevant until Level 3. Midgame damage increased. Veno moves from a support to a greedy support or midgame semi-carry.
6.80 - Once again, Veno's ability to support is nerfed. Moves further towards semi-carry.
6.81 to 6.84 - little to no change. Poison Nova ultimate improved due to DoT change.
6.85 - Massive damage nerf to Gale. Total dmg decrease from 25/500/975/1450 to 75/250/425/600. While this brings Gale in line with a regular ability (rather than an ultimate), it further hurts Venomancer as a nuker-support.
How is Veno used now?
In pro games, Venomancer has been used in almost any lane, from solo offlane, to mid, to safelane carry. He is rarely used as a support - and if so, he gets a Midas ASAP. One mistake some teams make when using Venomancer is not allowing him to get the levels to get his ultimate online soon enough, or to get enough levels to keep his ultimate's scaling relevant.
However, Venomancer can shut down many mid-lane farming carries with a mixture of Plague Wards and Poison Sting (poison sting can deal 32/90/176/288 after reductions, with Wards dealing 16/45/88/144 after reductions, over 6-15 secs). Unfortunately, Veno's own Sting DoT does NOT stack with the DoT from his wards, nor with itself (if you want an obscene stacking DoT, I recommend Huskar...)
Veno as Ganker
What's interesting about Veno is that, by Level 9 (Max Sting, Max Gale, L1 Ultimate), you can do 288+(600*0.75)= 738+auto-attack damage to one target with 1 cast and 1 auto-attack. Add a Shadowblade onto this, and you get 175 extra (physical) damage, plus the auto-attack (assuming attack damage of 84 with L9 and only the shadowblade, and enemy phsyical resist of about 25%), for a total of 932 damage (over 15 secs).
Veno as Support
The problem with Veno as support is that you need levels, and you're very slow and squishy. To me, this says "Blink Dagger", or perhaps "Midas into Blink". However, you still don't want to jump into the middle of the fray - just close enough to get off your ultimate, and then perhaps catch a couple of heroes with Gale. Throw down a few Wards to slow fleeing enemies, and your job is done.
As support, you will generally Max Gale and 1 point in Poison Sting, then Max Wards, and always prioritize your ultimate.
So how should I build Veno?
There are two popular skill builds:
1) Sting + Wards (anti-mid or support)
2) Gale + Sting (semi-carry)
Good Luxury Items for Veno:
Aghs: Ultimate damage is already disgusting. This makes it more disgusting, and reduces the L3 cooldown.
Octarine Core: Again, you're trying to get your ultimate cooldown lower, and a lower cooldown on both Wards and Gale is nice, too.
Shadowblade: As a mid-game item, you can gank quite effectively. L4 Gale damage is very high for a non-ultimate, and combined with the slow + Shadowblade bonus damage + auto-attacks, you can solo-gank quite well.
Boots of Travel: Veno also makes a respectable split-pusher, but I'd recommend a Blink Dagger or Shadowblade for escaping. BoTs allow you to be where you need to be on the map.