When Marine Corps pilots start taking to the skies in the F-35B this summer, their brand new aircraft will be lacking in quite a few areas compared to some of the aircraft that it is slated to replace - namely the A-10 Warthog.
Thanks to the F-35B's current Block 2B software, the STOVL variant of the F-35 will be unable to use night vision, it won't carry more than 4 bombs or missiles (and the specific types are also limited), and problems related to navigation and targeting software, flight controls, and the Automated Logistics Information System (ALIS) are still undermining performance. These issues are in addition to the general reliability and design issues with components such as the fuel tanks, landing gear tires, and engines.
Due to the Marine Corps requirement of a short take off, vertical landing (STOVL) design, the F-35B also carries about 30% less fuel than either the F-35A (the Air Force variant) or the F-35C (the Navy variant). This gives the F-35B only about a 30 minute loiter time over a target area, although the USMC intends to use the F-35B at dispersed, forward bases (known as mobile forward arming and refueling points (M-Farp)) to take maximum advantage of the STOVL design.
The Marine Corps is planning on declaring initial operating capability with Block 2B software in July 2015, when a USMC squadron with 10-16 F-35Bs is trained and able to perform Close Air Support (CAS), Offensive and Defense Air Combat, Air Interdiction and Escort, and Armed Reconnaissance with Marine Air Ground Task Forces. The Air Force is waiting for the capabilities present in software Block 3i, which is expected to be between August and December of 2016. The Navy is planning on declaring IOC sometime in late 2018, with software version Block 3F.
While software Block 3i doesn't add much in the way of tactical capabilities compared to 2B software, it does provide support for a upgraded Integrated Core Processor - the heart of the F-35's computer system. This upgrade should have a positive effect on some of the software problems and limitations that have cropped up in testing. Block 3F software should give the F-35 100% of its designed tactical capabilities, with better targeting, imaging, and data link abilities, as well as the ability to use all weapons that the F-35 has been designed for. There is also Block 4 software planned, designed to increase the F-35's weapons envelope and counter air and ground threats that are expected in the 2040's and beyond. This upgrade is planned for the early 2020's.