Tedious boss battles can wear a solo player down.
Sure, you can take out critical spots of the boss (usually fuel tanks), and this does shave off a few minutes off of a boss's lifespan.. but it's just not enough. These are the worst boss fights I've seen in a Kojima-led Metal Gear game, without a doubt.
It's clear that Konami intended a great deal of this game to be played co-op, as none of the missions scale in relation to the number of players present. Why was the game designed this way? And where is the Infrastructure mode that was present in the previous Portable Ops? 20gb PS3 users (like myself) and people who don't own PS3s at all won't be able to use the Ad Hoc Party system, either.
Another problem with the game is that is gives you little to no explanation about how various, and important, game functions work. You'll have to hit the forums to see various theories on how to grow your Mother Base plant count, for example. Was this done to sell more strategy guides?
Fortunately, the game's story and writing are quite enjoyable. You can listen to optional audio tape logs that flesh out the characters and world. It's great listening about Costa Rican and Japanese anti-militarism politics, the idea of nuclear deterrence, and Existentialism. It's just a shame the gameplay itself can be such a drag when played solo. If it wasn't for the strength of the Metal Gear setting, characters and writing.. I'd probably give this game a 7.0.