-- A specific example is "hardlight", which isn't explained in the game (maybe in the Greg Bear books?). Indeed, the first I ever heard that term was in relation to the cancelled MMO. I think it's some form of force field that the Forerunners use for all sorts of things - but why do weapons shoot hardlight?
-- The size of the Infinity is inconsistent (notably, when it rams that Covenant cruiser in the Ops video). The Forerunner glyph system no longer appears to have any reasoning behind it (when they exist at all now, maybe when the Foreunners started bevelling edges they changed their writing system). What happened to the mini-constructors from Halo 2? Those were cool. Why do all the grunts look totally different from previous games (in-universe explanation)?
Hard light isn't explained in-game but it is self-evident, it is hard light, duh (interestingly, there is a real theory that states light might act as a solid, google "Solid Light"). It can be shaped, so it is ideal projectile, good against the Flood. Of course this leads to a question, why not just use high power laser?
A X-Ray laser would have a range of 10 lightminutes, at that distance it would kill everything with radiation, at one light minute it would melt steel.
The size of Infinity is not inconsistent. It is long, not that wide (compared to its lenght, and tapers towards the bow), the CCS-Battlecruiser is under 2km in length, and not very wide either.
Though i'll note that the cruiser the Infinity rams in that one cutscene may not be a CCS-class, it looks so large. It might be something else, think the CSO-class supercarrier in Reach, looks like an assault carrier but is 5 times as large.
(If anything, the size of CCS cruisers is wrong in this game, i spotted one that is really small, like the size of a Falcon (glitched backround) in mission 5)