My personal opinion is that the ideal first client has three features:
(1) Starts up and is usable within a couple minutes (even 10 min the first time would be okay, to sync block headers)
(2) Supports Windows, Linux and OSX
(3) Uses deterministic wallets that can produce a permanent backup (preferably paper)
Encryption is a major upside, too, but people new enough to Bitcoin that they need such a simple client, can survive without encryption (thye're not going to be holding a ton of coins) -- as long as they are made aware that they do not currently have encryption, and the associated risks (and other options).
I think it's extremely important that users have a clear way to backup their coins to offline media or paper, in such a way that they don't ever need to worry about it again. Not only does it give users protection against hard-drive loss, it means that they may find it again in the far future when they haven't used Bitcoin in 2 years, and it reminds them that they still have coins (and they don't have to type in 1000 private keys to get their coins)
For that reason, I think Multibit is an excellent choice. I haven't spent much time with it, but I do understand it to satisfy (1) and (2) clearly, and (3) may be happening in the near future (along with encryption). But I do wonder if it has enough staffing behind it to be the center of attention (no offense to jim618, but if this becomes the "de-facto" client for new users, we should make sure there's a lot of people available to support it -- what if a major security bug is found? how long would it take the current team to identify, fix and test that bug?)
-Alan