A leaked DOD Directorate of Operational Test and Evaluation report stated major new issues with the CTOL F-35A. A major issue the report cites is a lack of rear visibility.
"All four student pilots commented on the out-of-cockpit visibility of the F-35, an issue which not only adversely affects training, but safety and survivability as well. One rated the degree to which the visibility deficiencies impeded or degraded training effectiveness as 'Moderate;' the other three rated it as 'High' or 'Very High.' The majority of responses cited poor visibility; the ejection seat headrest and the canopy bow were identified as causal factors. 'High glare shield' and the HMD cable were also cited as sources of the problem. Of these, only the HMD cable has the potential to be readily redesigned. In three cases, student pilots explicitly cited visibility-related impacts that could be directly applicable to the Block 1A syllabus (a largely benign visual search environment); several other implicitly did so. One student pilot commented, 'Difficult to see [other aircraft in the visual traffic] pattern due to canopy bow.' Another stated, 'Staying visual with wingman during tactical formation maneuvering a little tougher than legacy due to reduced rearward visibility from cockpit.' - Operational Test and Evaluation Report, 2013
While avionic systems (e.g. DAS & HMD) will mitigate the effects of low rear visibility, the inclusion of both a conventional bubble canopy design and situational awareness enhancing avionics is preferable. Ensuring that the pilot has good visibility has been a mainstay of good fighter design for decades. The F-35 still has a lot of design changes (particularly software) to undergo but the lack of rear visibility will likely persist. Its odd this design oversight hasn't been mentioned prior to this report considering its presumably been part of the JSF design for a while. While this development is certainly a major concern, it is not worth cancelling the entire program over it.
As a side note, its important to bear in mind these reports are supposed to be hard on the evaluated aircraft. A problem must not be sugar coated so that the necessary action is taken to remediate the issue. Immature designs always have issues that have to be ironed out over time. The software issues mentioned by the report will likely be fixed given enough time in a similar manner as the F-22's initial software problems. Its also important to note these are new pilots, arguably test pilots have a better understanding of the limitations of the aircraft (spazsinbad, 2013). Furthermore, not all the complaints among the new pilots were unanimous e.g. touch screen interface issues (Defense News, 2013).
Image 2: F-35A cockpit (image credit: Darin Russell, retrieved via Code One)