I suppose the most important feature for me is all unread tweets, alot of clients limit it to the last 200 which is kind of useless for me. When I refresh in the morning I could have 400 unread tweets, the other night I had 700, so a 200 limit is useless, some clients let you load the gaps in the timeline like the official twitter app, some don't like echofon, I've only found 1 client so far the does all unread and thats Simplytweet, and while its ok it doesn't load all image previews in the time line, services like lockerz and instagram don't get a preview, so I guess I need something that does all unread and supports most image services.