Someone posted on a well known dota forum that it might be nice for some analysis of competitive lineups to be done, to help people understand the -cm process. Replied with this, and thought i'd leave it here too:
I've picked out a matchup between EnRo and MYM from the DCPS, from not quite 2 weeks ago. I cant claim to have any knowledge whatsoever of what was actually going through the players minds as they were picking, but I can at least try to justify what they were doing to help you understand the process a bit more :)
Replay if you wanna see it:
EnRo vs MYMEnRo Bans:

MYM Bans:

So EnRo (with first pick) ban slardar, morphling, storm spirit and nerubian assassin. Slardar has been seen quite alot recently, and MYM had been utilising him reasonably well, so good to get him out of the way - he can be a bit of a nightmare in the laning/ganking phases if he's paired up well, and carries nicely from the mid-game with a nice bit of early farm. Morphling is a pretty nasty carry, and has the added bonus of soloing well to help him level nicely whilst farming, banned alot recently - not much to say here. The storm spirit ban seems rather tailored towards playing against MYM, as Angel has played him (and owned) alot recently, so rather than trying to counter-pick (with perhaps doom or seeker) they decide to simply ban and remove him. Nerub seems a bit of a mistake to ban here IMO, MYM aren't really known for utilising him perfectly and more often than not just leave him in the pool rather than taking him. If its a hero that EnRo don't like to play against though, then thats a fine reason to remove him from the pool (though personally I think SK would have been a much more sensible ban as MYM|Mania- is known for destroying games with him).
MYM remove dark seer, tide hunter, panda brewmaster, zeus. MYM pretty much ban seer every game - if theres a nasty hero which you dislike playing against, yet equally dont like to pick, just remove him from the pool. Seer is more than just an annoyance, it can spell disaster if he drags your whole team together in preparation for some AoE nastiness, his ulti is a pain when you're pushing, and ion shell is ridiculous for early laning and ganking attempts. Tide quite often is left unbanned and also not a definate pick when he's left in the pool either, but his initiation is rather nasty and so a ban which doesnt really need much explanation. Panda is another that goes without saying, at the moment he's pretty much the #1 desired hero in lineups, once he gets aghanims up his ultimate cooldown is crazy, and when played well can be a complete nightmare - the opponents have first pick, and MYM cant really afford to give them panda = banned. Finally, there are quite a few nice high priority picks left in the pool including rhasta, and what i like to think of as the 3 musketeers of support-gank - maiden/lion/venge, lots of teams can be built around these heroes and MYM ban zeus so that they leave some of these heroes that they love to pick in the pool.
EnRo Picks:
Rhasta is a pretty standard first pick, he is excellent when it comes to pushing, has alot of disable capabilities, can be played in a variety of roles from full on support, to a soloing pseudo-carry. All-round good, versatile choice.
MYM Picks:

Lion and Sanking, sensible picks again, heroes with ganking capacity seem to be the most saught-after picks this version, and both heroes provide this in buckets. It doesn't really give anything away about their intended lineup yet - ganking is the norm at the moment. With her recent soar in popularity I can imagine quite a few teams would have picked up venge at this stage, but leaving her out is fine too.
EnRo Picks:

Potm and Maiden, again heroes which are versitile within a team and don't force you into playing a specific lineup or anything - they are still capable of using their later picks to counter anything MYM do. With potm you get a great all-round hero, she ganks well, performs well in a dual lane but is also fine to solo, and provides you with some nice DPS late-game. Maiden is taken for her global mana aura, and will be the full on support hero of the lineup, she is a nice harasser in lane and plays the babysitting role to pretty good effect.
At this stage MYM can presume that maiden takes on a full support role, so its plausible that rhasta will be played in more of a farming role and may even take their solo lane. Though they have potm, they still lack dps, so it can be assumed that they'll pick a more full-on carry with thier last picks.
MYM Picks:

MYM pick up admiral and warlock. I dont know whether kunkka was an intended pick from the start (maybe a toss-up between him and spec for the carry role), but taking him provides the added benefit of preventing EnRo from picking him - remembering that EnRo probably want a carry for their lineup, and kunkka would synergise well with maiden. MYMs lineup is now pretty solid with lots of nice AoE and initiation as well as great ganking power. This prevents EnRo from jumping out and suprising with pushing heroes with their last picks (which could have been plausible, with rhastas ultimate and maidens aura), as it would be countered by AoE.
So from here on, EnRo can pretty much assume that MYM are laning SK and lion together, with kunkka and warlock together, and their final pick will be a solo hero that kuroky will probably play. Admittedly some teams have been using warlock as a solo recently, and kunkka could be laned with lion, leaving the sk to possibly pair with their final pick, but that seems unlike how MYM tend to play, and so somewhat unlikely. So EnRo are probably thinking that their lanes need to be strong to stand up to MYMs, they could do with more AoE power to turn the tide of the teamfights (which look significantly in MYMs favour at the moment), and they need a decent dps hero, possibly also with some AoE capacities (kunkka would have been great).
EnRo Picks:
Shaker and Davion are picked. Some greatly needed teamfight AoE is provided by shaker, who lanes perfectly with potm (always have to consider the lanes whilst picking, not just taking good heroes), he also ganks well and so can move around between lanes giving potm extra chance to level and farm alone. Davion is an interesting one, hes not a usual pick by any means, but in this situation he does fit the bill pretty well. They require a solid carry hero that will be of use in the teamfights (splash damage?), and who will be a good lane partner to maiden. There could also have been a consideration of playing a tri-lane of maiden/shaker/potm and then picking up a soloing hero to carry, but that doesnt really make any sense because MYMs dual lanes a strong can can both survive a tri-lane, and dominate a solo lane... so it doesnt really work. Davion is a fine choice, and probably better than other options of say spectre (slardar wouldhave been nice here if not for the ban).
So now EnRo are done, MYM can see that they will be soloing their rhasta, have a very strong dual lane (potm shaker), and a nice babysitting lane (maiden davion) which has some survivability against other strong lanes. They will have to watch out for ganks, because potm and shaker can be very nasty moving around when coupled with maidens mana aura. Looking at the lineups, what MYM really want is a solo hero, preferably one that can end up laned against the rhasta and beat him, and preferably one that has a bit of carry potential (all they have is kunkka so far). Things that come to mind are Tinker, Viper, Bane, SF and QoP (morph would be a good choice too if not for his ban).
MYM pick:
Perfectly fair choice, with the added capacity for even more AoE (seems its what they all want in this game). He carries nicely, solos well and with the proper support from his team, shouldnt get ganked too heavily.
To note: no venge or abaddon picks, these guys are usual up near the top tier - but as the picking progressed they didnt really fit with the lineups, so werent picked.
Anyway, hope thats somewhat of an insight into why the heroes got picked here... my attempt at being helpful.
Some things to keep in mind whilst picking heros:
1. Hows it going to lane up?
2. Which players are capable of playing which heroes?
3. Do I still have choices available to counter their lineup, or am I stuck to one route of hero choices?
4. Is it obvious to the opponents what I plan to do?