I have a wishlist of features I'd really really like to see in the official client, so I wouldn't feel the need to use an unofficial one.
1. Toggle to display object damage. Or a configuration option. It should stay set/unset. Ender's way of displaying this is fine, though it's a single control affecting several different displays.
2. Toggle or configuration options to display crop state, e.g. 1/4 for stage 1 of 4. Trying to remember the subtle differences in appearance is really annoying. Ender's way of displaying this is fine.
3. Ability to increase lighting level overall. I don't *think* the official client has this, though perhaps I simply failed to locate it.
4. When you get an hourglass (task is taking a while), show a % complete.
5. Ability to display a scrollable list of everything in a container, including your inventory, that's sorted by quality within type, and that flattens bundles. You can then use this to select a single item. Thus you can easily pick either the best or worst of a particular type of item in the container. Ender's version of this is good enough, but irritating for containers without much height.
6. (not in ender) Show me what kind of wood a given log is made from; don't require me to memorize individual appearance. I get wood type for blocks. I can get type for trees with examine. Why not logs?
7. the icon selection screen should be capable of grouping the icons, e.g. ender's "critters", "rocks", etc.
8. (not in ender) icon selection should be saved per toon, not per account.
9. There should be two "show radius" toggles. One for mining support radiuses, and the other for farming radiuses. The one for mining radiuses should include the support radius of the initial mine ladder. (Ender has that; official doesn't). Farming (ender only, but uses the same toggle as mining) should include mound beds as well as skeps.
10. Tool tips should give names for anything that has an icon on the mini-map. Flags (and only flags) seem to work this way in the official client. Ender used to have this, but replaced it with always-present text which just clutters the screen.