Note: All the following comments are from someone that loves and is addicted to objective gametypes in Halo 3. I've sunk exponentially more time in the objective playlists than slayer playlists. IMO, Halo 3 (all Halo's actually) are at their best in objective game modes.
The only point I really agree with you on is the third point. 3v3 anything on a large map is a pain in the ass, and putting any objective on a map that's not suited to it is just stupid. Asymmetrical gametypes on asymmetrical maps, two-way modes on symmetrical maps.
I used to think the maps in Halo 3 were a big disappointment, but they've all grown on me over time. Setting aside the remakes of Bungie's best (Last Resort, Valhalla, Avalanche, Blackout....I wont include Guardian, cause I hate it), there are still lots of great maps here. I loved High Ground from the get go because I love asymmetrical maps that force each team to use different strategies, but I hated Narrows, The Pit, Sandtrap and Snowbound at first. Now I appreciate all the little nuances of their design. You're right that the map design has only been getting better, though. Standoff, Rat's Nest, Ghost Town and Longshore are all fantastic.
At launch, the AR start hurt on bigger maps like Valhalla because you were basically helpless against BRs at mid-range. That was corrected with BR starts on the huge maps. Smaller to mid-sized maps like Narrows and High Ground still work with AR start. There's enough cover (either through different elevation, or structures, or buildings) that fire-fights typically start at or just outside AR range, which means use of grenades and melees even the score between BRs and ARs. Hopefully Bungie acknowledges the changes they had to make in Halo 3, and offers DMR starts on the larger maps in Reach.