We recently purchased 5 brand new EX4300-48T switches to replace EX3300-48T switches we have in service (mainly because of the dual PS capablities in EX4300 not present in EX3300).
All of them came with Junos software version 13.2X51-D26.2
We updated one switch to 13.2X51-D35.3 and manually replicated exactly the same configuration we have in one of our EX3300 which have been working OK for 2 years so far.
Result: Same cables of circuits in service that work OK in EX3300 don't get light in any port in the EX4300.
Then, upgraded the EX4300 to 14.1X53-D30.3 and got exactly the same weird behaviour.
Restored the configuration to Factory Default and created only one VLAN using ports 36, 37 and 38 which are ports with default speed, duplexing and auto-negotiation (all in auto) and for my surprise, when connected a cable to the port 38, got no light either!!
Tried disabling / enabling the port and same results. When I return that cable back to the EX3300 it works like a champ.
Got a second EX4300 of the 5 we purchased and upgraded the software to version 14.1X53-D30.3. connected same cable that works in EX3300 to EX4300 and no light either.
I double-checked the port where that cable is connected in the EX3300 and has speed, duplexing and negotiation all as factory default: in auto mode.
We have run with Juniper Switches EX3300-48T and EX4200-24F for years and no surprises so far. All of them (15 in total) working solid.
What's wrong with these EX4300 or what are we doing wrong ? What's different here versus Ex3300 and EX4200 in terms of configuration ? This is getting me nuts... 
Please HELP!
Thanks,
Javier.