We all know that it's not good to pick all Agility or all Strength heroes in a serious game. The reason for this being, these heroes don't have the raw power that INT heroes are able to achieve with nukes early game. More importantly, they are not reliable early game (more explained later on in the post). Most of them can really be set back in the game if they're ganked a lot. In serious games, we need heroes who are an asset to the team even with sub-par items because you can't run around site-seeing in these games - you're bound to get ganked.

So the best way to maximize efficiency would be to have a blend of INT/STR/AGI heroes. Ideally, we would like to have two nukers, a carry, a tank, a tank-carry, but this can change depending on the situation and enemy picks. That said, those two nukers don't have to be strictly early game. INT heroes like QoP or Nortrom can really carry the team if it needs one, so they're really flexible. An ideal combo is called a 'Lineup'. Popular double lane lineup combos include: Leshrac-SK, Lina-Crystal Maiden, and Earthshaker with just about any hero who can deal damage quickly (Juggernaut, for example)

Popular lineups used in league games are: Leshrac, SK, QoP, Spectre/Rikimaru, Bristleback. Axe is also starting to get popular these days. The reason heroes like Mortred or Troll aren't picked very often in serious games is that they aren't a reliable source of damage. The key word here is reliable. While they are certainly powerful when trading hits with heroes, you can't follow this idea in serious games. In serious games, you need damage that you know will DEFINITELY work, and that you WILL be able to execute it (eg: nukes, fixed damage attacks, AoE attacks)

Performing Combos
When performing a combo you have to talk with your team about who does what first. Initiators are the MiPs (Most important player :P) here.

Let's take the case of a Magnus-Warlock-QoP combo. The best way to perform the combo would be - Magnus blinks in, uses Reverse Polarity and tries to get as many heroes clustered together. Warlock then uses Fatal Bonds (damage shared partially between linked units) to link the heroes and summons Infernal. Magnus uses his Shockwave.
Right now, the linked units receive partial damage each, i.e, each of them gets 175% damage. Since all of them are clustered together QoP can easily finish them off with a Scream and a Sonic Wave, obliterating them.

[Note: The reason we don't get Warlock to use bonds before Magnus casts his ult is because when enemies realize that they're linked, they'll start to spread away from each other, and we definitely don't want that because it'll become difficult for Magnus to catch them all in his ult.]

This was just an example, and the combos you can perform in DotA are infinite. The only thing you need, is coordination. If the Magnus-Warlock-QoP combo had been performed differently, the results wouldn't be enough to kill all the heroes, or with maximum efficiency. You need to know your place. For this you need to talk a bit with your allies.

Countering Combos

Countering enemy hero combos is an important aspect of winning the game. Even a small change in their attack pattern can be very beneficial to your team, allowing you to push further or mess up their entire strategy. You have to understand the role of each hero in the enemy team. Human psychology tells us to dish out all our spells on the main enemy hero. Is this correct always? No. More often, it's the initiators and disablers that mess up our strategy; so we have to eliminate them first, so that we can focus fire on the carry later. Disablers are what help the carry to dish out his damage. Know where the enemy heroes stand and what role they're playing.

You can always pick counter-heroes with skills to counter your opponent team. That, or you can pick an all-rounder like QoP and improvise by buying items.

Here are a list of items that you can use to perform or counter combos.

1) Guinsoo's Scythe of Vyse - By far the best counter item. Can wreak havoc if used properly. Even the most powerful carry heroes can be brought down if you have a double-guinsoo combo on your team.
2) Blink Dagger - Every initiator's favourite item. Helps in loads of things, and the possibilities are endless.
3) Lothar's Edge - It's pretty useful on initiators after the blink dagger nerf in 6.48. Eg: Nevermore/Ursa.
4) Orchid Malevolence - Next best counter item after Guinsoo. 5 secs of silence is godsend to your team if you know when to use it.
5) Shiva's Guard - AoE damage is a definite must for every serious game. The armour bonus helps, but it helps immensely on AoE heroes like Puck/QoP/Earthshaker by supplementing their already powerful damage.

There are lots of other items, but I found these to be the most important in terms of combos. It's always good to have a Guinsoo on your team in every game. Spend time formulating and thinking about combos, it'll help in the long run. Definitely.