That's kind of the problem: Abilities that are just "more gun" are boring. Abilities that do damage in interesting ways (like Krieg's fireballs and explosions, or anything melee-based) are fun, but fall behind because their damage doesn't scale the way that weapons (and enemy health) do. That really just leaves utility skills being useful, but there are only so many ideas for utility skills, and you can't really build a whole talent tree around that.
Now, saying this, I did like all of the classes conceptually in 1 and especially 2 (I drifted away from PS having only tried Athena and Claptrap and not getting far enough into TVHM to experience full builds, and I haven't played 3 yet). There were some neat ideas that I would have liked to dig further into, but I'm also not going to pretend that Gunzerking with Rubi+DPUH wasn't significantly stronger than anything else available, and scaling issues mean a lot of builds really struggled to be viable enough to be enjoyable (especially into OP levels). Some scaling adjustments would go a long way to improving the build variety.
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