When to pick Queen of Pain and when not to?

With a couple of games in captain mode I have realized that I almost always like to pick qop as a mid laner. She seems to be just too good to miss. I don�´t play it too often, because as a pretty new player I think it is too much responsibility to play mid so I�´d prefer to play some support heroes for now.

But my question is, what kind of opponent´s hero picks specially favor qop, and what not? Why? Is there any specific hero that can be countered by qop, and is there any hero that counters qop hard?

Well I can answer a couple of your questions I think;
Heroes that counter QoP (in general);
-Doom Bringer due to his Doom
- Silencer can silence you and also he steals your int so not that great for you.
- Disruptor if they have vision once you blink away, they will bring you back with Glimpse
-OD possibly due to temp int steal and also can hit hard.
-Anyone who can silence you pretty long durations.
- Nyx Assassin, can stun you for long durations, burn you mana and nukes you pretty darn hard.
- Puck can be a problem to lane against as he can Phase Shift harass ( Shadow Strike and has an escape method.

When to pick QoP;
-When the other team lacks disables/silences
-when the other team lacks a strong mid hero
-If they rely on slows to get kills, (blink away into trees and tp out)
-If they have a reasonably squishy line-up
-If your team lacks a mid hero AND ganker

I don't play QoP much nor pick her drafting wise either so pay attention to what other people say as it will be a lot more accurate.

QoP is an instawin mid against heroes like Sniper, Drow, Zeus... she really punishes noob heroes. She's good against squishy heroes without escapes - so she owns supports. Also is good against melee opponents as she can harass hard with Shadow Strike.

When NOT to pick her is, as others have said, when there are lots of silences or burst stuns that can kill you before you can blink out. Believe it or not Bloodseeker is a counter to QoP, with his silence and Rupture punishing blink.

I think qop is more of a when to pick kind of thing, she is great vs heroes that don't have escape methods, squshier heroes and is really amazing when she has setup for her ulti in teamfights.

When you pick qop you need to build the rest of your team around her a bit because if anyone can get near her and stop her blinking away she is pretty dead. So basically you need crowd control, heroes like Axe, Clockwerk and Dark Seer are great with her.

I haven't played QOP much(only ~20 games), but I found out she is quite situational hero, as she needs both your and enemy picks to be quite specific in order to be effective before she gets some major items. Genereally, you want to pick QOP when your team has disablers and initiators, and when enemy team doesn't have them, and also doesn't have much silences.

Ideally, you want to have someone in party with you to pick a good disabler, bonus points if his disable is AOE, more bonus points if he can roam, more points if he scales late-game, and we get Sven. Really, they are very good partners, frags guaranteed! Other guys like Sand King, Centaur Warrunner etc. will do fine too.

The most feared QOP's enemies from my experience are Silencer and Nyx Assassin. Sadly, they are often there as a counterpick to you if you picked QOP so it's recommended not to pick her early. In CM you can just ban those. Other silences or disables can be countered by BKB or Linken's - one of these is 99% core, usually BKB.

When the enemy team has no escape. Your only form of disable is a slow, so heroes like Storm Spirit can easily dodge your combo and possibly **** on you. Any hero with a silence can kill you easily, so watch out for those.

Pick her when:

Enemy team doesn't have instant silence/disable (lion, rhasta, sky, doom, NS, BS) or ensnares (naga, ember, tree, meepo). Projectile stuns can be dodged and are less of a concern. Silencer's not a problem since you should be getting BKB anyway.

Enemy team has farm dependence as well as gankable heroes (low mobility and/or low HP).

Your team has a lot of disables but not a lot of magical burst.

You don't know how to play puck.

Honestly I'd say avoid Viper, outworld devourer. Both hit her pretty hard and can punish her, and are the epitome of 1v1 bullies. A good Invoker will bully you out too, QE with forge spirits and Cold Snap specifically (QW to an extent but you shouldn't be getting bit by his combos)granted you can dish out harassment back, and blink away, but he has passive regen and can constantly push you back if you're over aggressive. Razor is another bully, Kunkka as well. Qop can handle most heroes 1v1, just takes a bit of a swift hand to wield, you will almost always have the mobility advantage (minus against puck and storm, ember as well). Time your harassment and farm, while utilizing her solid range to keep away from as much harassment as possible.

Queen of Pain is a really good ganker and snowballer, but she can really fall off in the late game unless you end it fast. BKB makes her pretty much useless, and once heroes start getting significant HP, everything but your ult is pretty negligible.

Don't pick her against disable-heavy lineups, teams with a lot of tankiness, or Nyx Assassin. He is specifically made to kill heroes like you, so watch out.

Usually if things went well she has her core(BKB/Linken's & Hex/Orchid) by the time TЯUE carries start owning her so she can start bying some rightclick items - usually MKB, Daedalus or Mjolnir(or Rapier) to catch up. Not to mention BKB's are not eternal and start wearing off if bought as first major item so her magic damage is not obsolete quite yet(especially given mana regen, hex/orchid and some squishy and tasty support)