Sergio Perez joins the fold with Jenson Button firmly establishing himself as the lead driver, both are known for their smooth driving styles which leads to better usage of the slick Pirelli tyres. Sergio will require a little while to settle in perhaps changing ethos to the Sauber way of doing things. This will be mostly seen by Sergio himself as he adapts to a new tool unavailable at Sauber: The Simulator.
Only Lotus thus far have beat the Woking based team to revealing their car but just as I suspect McLaren will do today many details were not as first seemed.
Much was written last season about the effects of step noses up and down the grid and McLaren's adoption of what most fans would deem a regular nose. As the season progressed the team however made adjustments and by Mugello they had settled with a half way house concept which saw the nose tip raised in order to flow more air under the car. Why didn't they just switch to a step nose? many asked, the nose however was not the limiting factor but the chassis itself with the team not running to the maximum height restrictions as others had. It's not as simple as bolting one part from another car onto the car in question and the team must evaluate many factors both upstream and downstream of the component before introducing upgrades.
In 2012 McLaren arrived at testing and Melbourne with what most considered to be the most effective exhaust solution now known by most as the 'Coanda' exhaust due to the way it utilises Henri Coanda's principles of introducing a jet of air to manipulate airflow in the region. Most of the field converged on the McLaren ethos whilst Red Bull forged their own path. As it became apparent toward the end of 2012 that their Ramp and Cross-under tunnel now provided the same if not more downforce than the McLaren solution teams will have started to assess exactly that.
MP4-28
When the drivers peeled back the cover on the MP4-28 launch you could be forgiven to looking at it and thinking it's an MP4-27. Just like the Lotus launch I think we have to take some of what we see with a pinch of salt, it would first appear that McLaren are looking once again to dismiss the step style nose but in fact they have gone more towards the limitations of the 625mm bulkhead height and used the modesty panel to cover up any discrepancies. The Front Wing pylons although they look the same dimensionally longitudinally appear to be pronounced from the nose as the under belly of the nose tapers toward the front of the car
They have also followed Ferrari's path by adopting pull rod front suspension which undoubtedly help the team to unlock the Pirelli tyres more effectively whilst also marginally lowering the Centre of Gravity.
The Sidepods have been treated to a new design too with a step created in the bodywork to define the regions where airflow travels over the Sidepod (With the section above the inlet slightly ramped) and the undercut starting further back. This is so the crash sturcture element is offset to the inlet a design that Lotus used on the E20 and R31 (Renault). A very simplistic airflow conditioner is also attached to the side of the sidepod departing from the L shaped wing conditioner we saw toward the tale end of 2012. This is certainly one area where I believe McLaren are being coy at launch and we will see changes at Jerez/Barcelona/Melbourne. The Bargeboards have been amended in order to facilitate the airflow amendments needed by the re-profiled Sidepods along with what looks like a change in design to the front corner of the floor. The mirror stalks also serve an aerodynamic purpose, purposely blade shaped in order to adjust the flow in that region.
Further downstream and the Sidepod appears to have a similarly steep curvature leading to the exhaust trough as the solution ran on the MP4-27 from Hockenheim onwards. The exhaust undercut also looks very similar to that specification along with the exhaust positioning itself. It remains to see whether the car will be the same come launch in this area but I'd expect a significant alteration. 
One of the key features of Adrian Newey's since the inception of the new rules in 2009 has been the isolation of the halfshafts from the airflow regime by the utilisation of a shroud. In free air the halfshafts rotation with create downforce (Magnus Effect), the exhaust plume however has an eratic airflow and thus will create an issue when providing airflow over the floor of the car impinging on the halfshafts downforce generation. McLaren have now followed Newey's lead and isolated the halfshaft by means of a shroud too making it more simplistic to model and provide uniform downforce levels.
The rear wing is most likely a placeholder like the front wing but as we can see the top flap on the launch car is extremely short chord with an extreme AoA on the mainplane. No DRD is in sight on the launch car but I'm quite sure the team will be assessing it's potential especially with Lotus having said they will continue to work on the passive drag reduction device.
The long and the short of it is the MP4-28 is a new car, different in ethos to it's predecessor and will likely feature many more components at testing not shown here at what is essentially a marketing launch. Major omissions include under nose turning vanes and rear brake scoops which will likely form areas in which the team have spent development time on. (Lotus also excluded under nose turning vanes from their launch car / renders)
EDIT: Based on the 3D render made available on McLaren.com there are a couple of discrepancies just like the Lotus launch with McLaren showing the fin style Front Wing used towards the end of the season and Sidepod horizontal flow conditioners (Leading Edge Slats) above the inner ramped section above the sidepod inlet
If you'd like some other perspectives on the MP4-28:
Craig 'ScarbsF1' Scarborough Craig 'ScarbsF1' Scarborough with Peter Windsor filming at the launch
Gary Anderson (BBC's Technical Pundit and ex Jordan/Jaguar Engineer: http://www.bbc.co.uk/sport/0/formula1/21274591 (Grey bar on the left side of the page)
Mark Gillan (Ex Williams) - http://www.jamesallenonf1.com/2013/01/fastest-car-of-2012-made-even-better-technical-analysis-of-new-mclaren/