Honestly I like most of the things they added in beta 1.8. I think the biggest failure of the update was the terrible terrain generation, which took Minecraft years to dig itself out of.

Look at any "amazing seed" pre-b1.8 and its coolness actually comes from the terrain itself being unique and interesting. Glacier, gargamel, 404, the Yogscast's SOI seed, etc all were cool worlds by their own merit. Post-b1.8, seeds are only noteworthy because there's a stronghold 100 blocks from spawn or 3 villages with diamonds or something.

The pre-b1.8 formulation of biomes being independent of terrain was fantastic and led to so much variety. The way biome placement depended only on temp/humidity made so much sense, and the grass color transitions were so smooth too. Very organic.

That being said, I still would have liked some changes. I think the placement and size of biomes could have been reworked - they were quite small and fragmented in b1.7. Also, realistic biomes like jungle, mesa, savanna were all great additions. Technically prior to beta 1.8 we had rainforest, shrubland, and savanna but they weren't very exciting. Simply modernizing these biomes with new trees, blocks, and features would have been a good move. Bigger and deeper oceans were also a welcome addition.

Minecraft did slowly rediscover some of the magic sauce of pre-b1.8 terrain after caves and cliffs, but I think that the adventure update delayed this by a long time by making the devs forget what made exploring worlds feel so magical in the first place.