This was my thought too (though I'm hoping for a 125 achievement), getting to 100 even with the 2 mount discrepancy is pretty easy, at least I think you can get it without needing any rare drops or purchased mounts.

@greenbeans

Alliance has:
40 Faction Mounts (including flyers)
8 Talbuk mounts from Kurenai (takes about 8 hours of grinding after you do all quests)
4 mounts in Dalaran (one costs JP as opposed to gold)
1 Winterspring mount (about 20 days of work to obtain)
14 Argent Tourney mounts (9 are 100+ badges, pretty sure you can only get about 25 a day, so you're looking at months of work here)
6 Netherdrakes from Netherwing Faction (takes about 6 hours to get to exalted)
5 nether rays from Shat'ari Skyguard (another 6 hour grind)
1 Cenarion War hippogryph (this grind sucks, takes about a week due to heroic lockouts)
2 mammoths from sons of hodir (another lengthy grind)
1 red drake from Wyrmrest (mild grind if you grab a tabard)
2 camels from Ramkahen (all too easy)
9 PvP mounts
2 talbuks from Haala (requires the opposite faction to grant honor when you kill them)
2 Archaeology mounts (good luck)
3 Tailoring mounts (requires being a tailor)
3 Engineering mounts (2 require you to be an engineer)
1 Alchemy Mount (purchasable on AH for 50k+)
1 Seahorse from quests in vash'jir
-----------------------------------------------------------
105 mounts with significant faction grinding, daily questing, and PvPing, not to mention having to be a tailor and Engineer. Paladins get 3 more mounts, Warlocks get 2 more. Without Engineering and tailoring (and not dropping crazy sums of gold for 3 of the mounts) you're looking at 97 mounts +1 for the 50 mount achievement, so 98 total. This means that Alliance needs 2 achievement, Blizz-store, or rare drops (this includes Green proto and Polar bear mounts) to hit 100 mounts. Horde has those 2 mounts with the same effort. This is why Alliance players were/are upset about not getting faction mounts for the Worgen at release. Currently the Alliance has no reason to get exalted with Gilneas other than the rep achievs and the Ambassador title.

I can't get the Dark War Talbuk. It is purchased with tokens you get from killing the opposite faction in Halaa, but the Horde won't come out and play. /sad

@greenbeans - I'm almost at 100 mounts, and the only rare drops I've gotten is the Green Proto-Drake and the Raven Lord. Since I still have four more mounts to go on the Argent Tourney I can attest to being able to get to 100 without any rare drops.

@Natsumi - it's actually only 14 Champion's seals a day for the Argent Tournament now (they removed them from ToC and ToGC). I would have loved to get 25 per day. Right now it takes exactly a week of Argent dailies every day just to get a 100-seal mount. I should know I've been working on it every day for the last two months.

@ Natsumi

You forgot a few:
-the White Hawkstrider (which is a drop, but the drop rate isn't bad),
-Blue Protodrake (drop is pretty bad)
-the AQ mounts (all but 1 drop constantly--and yeah, you can't use them outside but they still count)
-Headless Horseman (drop is way better now)
-2 Brewfest mounts (fairly decent drop rate)
-Violet Protodrake from holiday achieve

I understand these are somewhat rare drops, but most mount collectors have all or most of these.