What I Liked
In true Google style, Keep It Simple Stupid is in effect. The client is incredibly lightweight (900KB install) and the GUI is very unobtrusive. The windows have little or no border, and nothing is there that doesn't need to be there such as ads, stock tickers, etc. The client is extremely lacking in options, although this might be a good thing to most people. Right clicking anything reveals few if any available actions. The preference screen has only the basics, but most options that I usually set on other clients are the default here. Not playing a notification sound while the window is visible is one of those features you think would have been the default for years. I still want to strangle people who can sit in front of the AIM client talking to one person with the conversation window maximized, and still need to hear the chime sounds for both sending and receiving IMs. These are usually the same people who still have their desktops set to 800x600 (once saw 640x480) even on a 19" LCD. That’s another story though.
Multiple conversations are stacked together and can roll up to be hidden if the need arises. The main buddy list window can be minimized to the system tray if you don't like it taking up room on the screen or taskbar. The client also acts as a GMail notifier so you don't need to have both that and the standalone GMail notifier. In conversations the “screenname:” stamp doesn't repeat itself if you've sent multiple IMs without alternating to the other person first. This makes sense but still throws me off a little. The away/available status is easy to flip and easy customize with no dialog windows and confirmations to click. Also there is a quick filter/search box for whenever you eventually get so many buddies that you may have trouble finding them by scrolling. Like everything else this is quick and intuitive.
The voice chat feature could not be easier. Even behind our NAT routers and firewalls, there was nothing to configure for either myself or my friend. The sound quality was crystal clear. There's not much to it, you talk, you listen, it works.

What I Didn't Like
I know Google was trying to keep everything simple as can be, but I would have liked to see basic file transfer, or inline images. Even if you don't agree on inline images, it’s the 14yr old girls that make or break an IM client, not computer purists or snobs. And the 14yr old girls want to share pictures on screen without any hassle. But this is a beta, so like the many important features of GMail that were added in later versions, we can expect Google to listen to its users on adding new features.
Another thing I see as an issue towards mass adoption is the close meshing of the service with one's GMail account. In order for someone to try out Google Talk, they must first sign up for a GMail account (if they haven't already). GMail is still in Beta, and if I've heard correctly still requires new users to be invited in by old users (correct me if I'm wrong). While GMail invites are cheaper than Soviet currency, it's still an extra step (or 3) to new users.
Also, your GMail becomes your screen name for anyone wanting to contact you. This has its advantages and disadvantages. It will make it easy for friends of mine to find me quickly. However, I don't always want everyone who knows my Email (which could be any number of harvesters or spam operations) to be able to IM me. In the blog game, one will often post their screen name for strangers to contact them about whatever, which is fine except that I'd have to post my personal email account on the web, which is something the web-savvy know not to do. My only option would be to have 2 Google Talk clients open (is that possible?) with different GMail accounts, one personal and one public. Perhaps Google could add a multiple account feature or allow non-email screennames or ID# to allow more anonymity? The Jabber protocol this client is based off treats your screenname kind of like an email address. For example it would see you as Someone@gmail.com, whereas 'Someone' can be anything you wanted, for the sake of simplicity Google makes 'Someone' your gmail account name. I think with a little tinkering on their end they could make user defined screennames (more than just the Alias field) that lets us talk without giving out our personal email at the same time.
My last gripe is the interoperability issue. Yes I can access the talk.google.com service from GAIM or any other multiple-service client. But what’s the point? Everyone I'd ever know on Google Talk would already have an AIM, Yahoo, or MSN account already linked to me in the first place. It's the interface that I like about Google Talk and if anything, I should be able to use my AIM account through Google Talk's client. Call me picky, but I don't like having multiple running clients open on one desktop. If Google Talk could cover all the other services, it would allow us to slowly wean ourselves off the other services to strictly Google (like my slow move from Hotmail to GMail) once its popularity has taken off and the program leaves beta, which is not for a while if its anything like Google's other services (Gmail, News, Maps etc).
I know some of these features would be against the main point of Google Talk and keeping it simple, perhaps there should be a Pro version along side that lets the veterans get what they want, without confusing the novices.

Conclusion
Over all, I think they are on to something and I applaud them for taking a shot at leading the way to a free and open standard for a medium that is illogically fragmented. The interface is great and I hope its shortcomings will be addressed in later versions.

Pros