You'll want to pick up the other expansions and add-ons. While it is possible to level up in the other areas past 75 without access to Abyssea, it'll be slow given that people will choose Abyssea parties over more "traditional" parties, so most of your exping would probably be done alone. Not to mention you'll want Abyssea for the sake of Empyrean gear and other assorted equipment that comes from the area, which are quite important pieces to have if you want to try your hand at some of the other content currently out like Voidwatch and Legion.
Fortunately, there are bundle packs containing all the expansions and add-ons that you can download or purchase for extremely cheap (like $20 for everything), so it's not anywhere near as expensive to get them now as opposed to back when you had to buy each of them separate.
Edited, Aug 2nd 2012 2:46am by Vlorsutes
My thoughts and reviews
Lady Jinte wrote:
Vlorsutes' Negotiation Skill rises 0.2 points
Vlorsutes' Observant Parent Skill rises 0.3 points
Vlorsutes' Argument Diffusing Skill rises 0.1 points
on all sorts of sci-fi stuff...and things.