Another analysis being clamored for - I will have it noted I have not played with Orks as a parent army yet, just as opponents and allies. You're own fault for demanding so loudly ^^. Let's hop in.
Like Tyranids, the biggest advantage for Orks revolves around the changes to Fearless and removal of No Retreat wounds. Although Orks aren't natively fearless like Tyranids (assuming synapse is around), at larger squad sizes one of their major weaknesses in 5th edition is simply removed. No longer do Orks lose combat and then lose a bunch more Orks and this makes large squads a bunch better in combat and as a tarpit (one of the reasons they make fairly decent allies). When you start dropping below 10 Orks though, expect them to disappear pretty quickly as normal - it's just when they are benefitting from the Mob Rule when this comes into play.
The improvements in combat against vehicles are also huge for Orks as it can really allow them to overcome some of their unreliable shooting. Previously, a vehicle could park itself in front of Orks and quite often survive the ensuing weight of attacks through sheer impossibility to hit. Now with Hull Points and changes to vehicles in combat, charging Orks will reliably wreck any rear AV10 vehicle. This takes a huge burden off the BS2 based shooting of Orks though the movement restrictions opponent vehicles can impose upon Ork hordes is still harder to overcome. Pile-in's can work in Orks favor and against them though due to their low initiative. If their opponent is able to wipe out a 3.1" area around their models the Orks cannot then pile-into combat and thus don't get to make any attacks. On the flip side, if the opponent is unable to do this, they generally bring more of their models into your grasp and your PK Nob is much more capable of getting into combat and swinging. Speaking of PK Nobz - great in challenges though still not fond of PFists as they still cause instant death - try to work combat so they get locked out of combat due to the same pile-in restrictions just discussed.
Disorderd charges are also a huge kick for Orks - no more multi-assaulting and getting S4 thanks to Furious Charge. The nerf to initiatives here isn't huge except Orky vs Orky games as I3 was still slower than all armies but Tau or Necrons (who will die anyway to Orks in combat). The lack of ability to assault multiple squads or a squad and a vehicle and still get S4 though is pretty big and is a limitation on the otherwise awesome bonuses to larger Boyz squads.
Cover also slaps these around a bit - no longer are they 6 point models which basically come with a 4+ save - the cover save is much more likely to be a 5+ now unless you play on ruin heavy boards or bring your own through fortifications (hello Aegis Defense Line). Even then, cover is determined per model and with the option to focus fire, Orks go down a lot faster than before. Also, as models are removed from closest to closest, this easier removal of models can drag the Orks further from where they want to be - assault. That being said, a Kustom Force Field is a great addition here as it still grants units cover within it's radius rather than models.
So Ork shooting... Well it's better in that taking out tanks is more reliable for everyone. All those twin-linked rokkits, big shootas, lootas and kannonz are now more likely to drop a vehicle in a given shooting phase which really helps the assault options actually engage something juicy rather than metal bawkses. The good thing with most of this shooting...is well it's kinda decent against flyers. You're not losing much in terms of shooting capacity (BS2 to BS1) and often have twin-linked or high rate of fire to accomodate this already. It's not great but it's certainly useful. As has been mentioned in the comments, this is great for heay weapons and moving as well - you're not losing that much in terms of chances to hit and you already have a high rate of fire to accomodate this... In reality though, this is where Allies will really come into play - Orks hordes are better and suddenly they can get good shooting and vehicles through allies. Crickey. We'll look at that a bit later though and cover...Kannonz now!
Speaking of Flyers... the Ork ones now aren't too bad. Not great but with good moderate-strength firepower and a relatively cheap cost + twin-linking, they are able to down other flyers and put out some decent anti-infantry on the ground. Not the best flyers around but not the worst either. That being said, only the Dakkajet should really be considered and with Allies, you have access to some of the best flyers out there anyway in Vendettas & Night Scythes.
Kannonz were decent and cheap before and with the changes to artillery (tougher), they are insanely better which brings up some issues if you're looking to use Battlewagons (4 hull points!) or Kanz. Battlewagons are really the only transports one should be using for Boyz now - Trukk squads are just too small and don't get that Fearless advantage. Trukks can still be bought for other squads - i.e. Nobs, Meganobz, etc. and be great distractions/mobility enhancers but there are even less reasons to take small Boyz squads when there is no direct disadvantage to being Fearless in combat. What a shame Phil Kelly thinks Xenos shouldn't be allowed to take transports the squads cannot fit in. Regardless, if you're not taking Nobz to access Battlewagons for Boyz and want them, you'll need to wrangle them out of your Heavy Support but then again, large 30 Boyz squads on foot work as great midfield control, particularly as Allies for other armies (i.e. Tau, Crons & IG). These squads are vulnerable to shooting and having the Nob not engage in combat so be careful how you use these but they do a good job of jamming the opponent. Kanz can still be quite scary though with only two HP and cover drop to 5+, aren't super durable, even on the charge some halfway decent overwatch fire can drop them but then they can be taken in squads (much improved in 6th) and like other Dreadnoughts, generally are quite happy to keep trying to launch long charge ranges. And the best bit...?
Waaaaagh! The whole army gains fleet and in case you haven't heard yet, Fleet rocks. Fleet through terrain is better than assaulting through the open normally and it makes all charges really reliable and longer charges a much more distinct possibility. For an army with lots of combat options this is a very good ability - unfortunately Ghaz's automatic 6" Fleet has been FAQ'd to be an automatic 6" Run - no advantage really conferred to the actually ability to fleet. Regardless, using this ability is very important - you can either get some units stuck in early and tarpitting the opponent or get a lot of improable assaults off. Be careful though, Overwatch sucks for Orks due to poor armor saves (though Nobz, Kanz, etc. care a little bit less) and this can reduce their charge distance and do more damage than good if you consistently fail charges. This is true for any charge, not just on the Waaaaagh turn and thus you need to be very careful with charge decleration (even if you fail you get shot) and squad positioning (i.e. not one guy sticking out or if one guy is sticking out, he is tough/has LoS!/etc.) become very important. You can't just keep throwing Orks into charges hoping they make the distance, you'll lose too many guys that way.
Nob Bikers as everyone is going on about the, got a nice boost as well with their toughness value becoming true T5. With a 12" move + turbo-boost, LoS!, two wounds and potentially very large charge ranges, they can really get into an opponent's grill early and be a right pain. They are very expensive though and die to focused firepower just like before - it's just harder to cause instant death on them. Burnas are now a really great charge deterrent thanks to D3 hits per model on Overwatch - it's not going to protect the entire army but it can make an opponent think twice about engaging them or trying to multi-assault them. How you apply this advantage on the tabletop though is up to you...
Which brings us to Allies...Orks have some of the best AoC for a Xenos army, right up there with Necrons & Tau. With access to Imperial Guard, Tau & Necrons, Orks are able to put a bunch of GOOD and RELIABLE firepower on the tabletop to support their crazy greenskin brethern throwing themselves at the opponent. Two of these armies currently come with good flyers as well so if you're looking for some help in that department and non-foot mobility, Guard & Necrons can really help out here (and still provide good shooting - seeing a trend?). Chaos are also an AoC but as they currently stand, aren't a great army so nothing to write home about. They don't cover the main weakness of Ork's with their poor shooting though this may change with their new book around the corner. With the aggressive nature of the Ork army the desperate allies probably aren't a great idea - they can't hold your backfield objectives and are going to be getting in the way of the Ork units pushing forward.
All in all Orks got a big boost both with changes to core game mechanics (i.e. No Retreat disappearing) and the ability to bring in some really good allies to shore up their codex and army weaknesses. Don't presume Ork Boyz are unbeatable in combat now though - they are much better since they don't keep losing bodies when they lose fights but they are still mass S3 attacks in protracted combats and with cover reductions, they are weaker on foot. It's a net gain no doubt about it but they aren't unbeatable (even Nob Bikers) - stop despairing!