So there's no point neglecting your collection now if you want to stay on top of things in future expansions.
I'll give you a simplified list. It'll be up to you to research details, though I may eventually post some guides for harder grinds. This is just to point you to the mounts that are guaranteed to be yours if you put the time/money/effort in.
- Battleground Mounts  - 50,000 Honor (each)
- Wintergrasp Mount  - 300 Stone Keeper's Shards
- Kurenai/Mag'har  - Exalted
- Netherwing  - Exalted
- Cenarion Expedition  - Exalted
- Kirin Tor  - Gold & 200 Emblems
- Oracles  - Revered
- Wyrmrest Accord  - Exalted
- Alliance/Horde  - Exalted All Races
- Argent Tournament  - Champion, Exalted, 875 Champion Seals
- Winterspring/Venomhide  - Exalted
- Motorcycle  - Gold
- White Polar Bear  - Hodir Pre-Reqs
- Heroic Culling of Stratholme  - Timed Run
- Sha'tari Skyguard  - Exalted
- Sons of Hodir  - Exalted
- Deathcharger  - Stratholme Boss Drop (directions)
- Time-Lost Proto Drake  - Rare Spawn
- LFG Random H-Occulus  - Loot Bag Drop
- Violet Proto Drake  - Get all holiday achievements.
- Tailoring  - Tailoring Level 425
- Engineering  - Engineering Level 375
- Fishing  - Northrend pools.
- Holidays  - Run relevant holiday bosses through LFG daily.
- 50 Mounts  - Get 50 mounts.
Partner Recommended (But Not Always Required)
- Halaa  - 170 Battle Tokens, 350 Oshu'gun Powder (makes 35 Research Tokens)
- Qiraji Battle Tanks  - Temple of Ahn'Qiraj, Trash Drops (directions)
Zul'Gurub  - Raptor & Tiger Boss Drops(Being Removed)
- Raven Lord  - Honored w/ Lower City
- White Hawkstrider  - Access to H-Magister's Terrace, Boss Drop
- Fiery Warhorse  - Karazhan Boss Drop
If any of my numbers are wrong, say so and I'll fix it. I have intentionally left out mounts that still need 3+ players as well as class-specific and out-of-game purchasable mounts. If you wish to keep track of your collection, I recommend the collection feature on Warcraft Mounts.