If you could skip all the trash and just kill the bosses...why not make 5 minute dungeons that are just 3 bosses...
The number of abilities is fine right now. We're only at level 50 with an impending cap increase (expansion). Also giving you a library of abilities and a limited number of slots as you suggested...SOUNDS good...but then you're just pigeonholed by the "best" ability loadout (see FFXI's Blue Mage and "x spell or GTFO") as determined by the min/maxers.