If we're including D/P mechanics... this gets extremely screwed up. I'm too indecisive... so here're some of each starter's uses, both new and old, in their final evolution forms:
Venusaur - The original super soaker, built to soak damage. Its reputation at bringing down opponents with mountainous HP and monstrous defensive stats over time is well known. D/P upgrades: not much, but Sludge Bomb is now special, so it hurts a freaking lot coming from a poisonous dinosaur.
Charizard - Famous for being the physical sweeper that cannot be walled once it sets up, and it does so with extreme ease as it learns the fabled Belly Drum. Its SP.ATK and being a fire type wards off physical walls. Some prefer Zard over the new member of the 600 club - Garchomp, simply because it tears Skarm a new one with relative ease. D/P upgrades: significantly, Dragon Claw is now physical, and the only thing that resists dragon attacks are steel types who gets melted by Zard's fire power. Bellyzard just got scarier.
Blastoise - Our resident UU bulky water is also the only starter - in fact, one of few - who can Spin and Haze seemingly well together. D/P upgrades: Blastoise has remained relatively unchanged, with the most notable addition to its learn-set being Flash Cannon... which is interesting rather than devastating at best. It does acquire some useful moves such as Fake-Out and Aqua Ring; both are more useful on others than this thing. Poor, poor Blastoise.
Meganium - Our friendly Johto dinosaur (friendly; even to the enemy) has taken it's kiddie gloves in exchange for some real punchers. D/P upgrades: In addition to Wring Out which compliments Meganium's weak offensive nature, Leaf Storm solves our friendly green long neck's petty attack potential to something everybody should look out for. If used together, it hurts, a lot. A great asset on any team, especially when using the wall techniques and paired with Choice Item users.
Typhlosion - The star sweeper of GSC. Although moving to D/P has made Typhlosion lose arguably it's best assets; the elemental punches (as they are now physical) GameFreaks took care not to disappoint the Zard-on-foot fans. D/P upgrade: Most notably, Eruption is now a learn-set. Throw a pair of Choice Specs on this bad boy and you've got yourself one of the hardest hitting attacks in-game. Paired with Focus Blast, this thing is a real game breaker; screwing over two major walls of the entire metagame.
Feraligatr - This little beauty learned enough new tricks to make Steve yell crickey from his grave. D/P upgrades: Almost everything was immediately a plus for our physical water friend. Among its arsenal of new attacks (since a lot of the attacks are now physical, such as Crunch and Waterfall) are Superpower and Ice Fang, both of which are physical and a welcome addition to the physical sweeper's learn-sets. Aside from physical STAB, it can now learn Dragon Dance to make use of these new found sweeping power; talk about over killing a buff.
Sceptile - The RSE Sub-Seeding Ninja seemed like it was nerfed upon learning Leaf Blade had become physical... but people soon realized it's new found potential. D/P upgrade: High speed + high special attack makes this thing the idea Choice Specs user, especially with Dragon Pulse at its disposal. New learn sets Night Slash/X-Scissor are useful when paired with Swords Dance. It's still the Sub-Seeder of wonder, just now you can expect it to do more than Sub-Seed alone.
Blaziken - D/P for this thing was all about buff buff buff. D/P upgrades: Agility, Baton Pass, Night Slash, Shadow Claw, along with Brave Bird/Flare Blitz, not only has its usefulness been shot through the roof, but its novelty value has reached the competitive level.
Swampert - The second, and best ground/water dual type to ever have been created. D/P upgrades: including the immediately noticeable Hammer Arm, Swampert can also pack Avalanche. Its also one of the most viable source of Stealth Rock, being only afraid of grass types.
Torterra - This is what happens when Venusaur gets freaky deaky with Blastoise: a natural born Curser capable of unleashing STAB Earthquake, all the while regaining lots of HP with Leech Seed. Many physical moves at its disposal.
Infernape - "Infernal Rape", fast and furious with a touch of ape. True to its fire starter kin, its power is acknowledged both in physical and special forms. STAB Close Combat with an above average speed that outspeeds many pokes. Also learns Thunderpunch, just in case a bulky water ruins your sweeping streak. Also one of the fastest Encore users, allowing easy setting up (Also learns Scheme, man there's no end to this dude's awesomeness)
Empoleon - "Emperor Napoleon", the name in no way suggests this pokemon's usefulness in battle, but it does clue you in on its status; royal pain in the butt. Water/Steel dual type allows it to survive fairly well on its mediocre defense. Slow speed compliments Brine and Avalanche rather well. Swords Dance gimmick set can be surprising and even viable for the first couple of turns. A decent mixed sweeper due to its decent SP.ATK.
There, I said way too much. But I had nothing better to do...