Yes I know what you mean about the linear side of MGS, especially in "The sons of Liberty" sometimes I feel like having bought an animated movie on dvd. I think the real gameplay time was shorter than in MGS1, and less possibilities to put the weapons in practice too...
Look, for example, you have to use the nikita only once during MGS2, and it is useless elsewhere, whereas it was used to open a gate and to fight Vulcan Raven in MGS1 and others cool things to do with, like tricking the guards.
MGS Sons Of Liberty was less fun.