Just so it's clear up front: I think Lion has a lot of interesting features that many user will like. What I write below is simply my opinion on Lion's negatives WRT how I use my Mac for work and fun.
I have been a part of Apple's Appleseed beta program since it began in March. I've been totally immersed in Lion as I've used it for eight-ten hours a day as my primary OS. It has resided on an OWC SSD inside my early 2009 Mac Mini.
. I absolutely detest the "versioning" feature in the Lion version of Preview, even though a couple of fellow commenters provided some interesting thoughts about it. My way of doing things since the 1970s is to save early and often, using the cmd-s key combo on Macs since day 1. While some applications will do an auto-save for you, these saves are the entire document. You can all go read on the Apple site how the new versioning works in Preview so I won't go into it here. My problem is that I cannot save a new copy to save my soul. Recently I needed to send a single page from a 20-page PDF document. I thought I had deleted all but the one page in Preview yet the person to whom I sent it asked why I sent everything. So Preview saved the whole document, not just the single page that I wanted. This is the tip of my war against Preview.
The new mail really likes to group threads together, even combining previously-read eamils that I thought I had trashed. When I trash an email, I expect it to be gone. Yes, I know that you can go "delete the email trash" somehow, but I really don't want old emails being resurrected just to join a thread. And yes, Mail has an option to have it behave like the old Mail.
3. No Rosetta
I know, we have to move forward so OS X doesn't get bloated with support for legacy apps but there are two programs that I use several days a week that are Rosetta-based, so I have needed to boot into Snow Leo just to perform some work with them. Please, no flames about this point!
It may be my older computer but when I run Lion for a day or two without restarting, overall performance gets jumpy. I'll be typing along when all of a sudden the cursor simply stops even though I'm still typing. After a couple of seconds, the text that I've typed will suddenly appear and off we go again. Other things that happen are slow window drags, slow scrolling (I use a scroll wheel) and slow window refreshes. I've opened Activity Monitor and that shows nothing hogging the memory or processor. According to AM things are working fine. I've booted on my Lion backup drive and I get the same results. Restarting clears things up.
5. Loss of color
I really like having colorful icons in my sidebar and elsewhere so this shift by Apple toward shades of gray is not to my liking. It's hard to distinguish among several objects if there aren't the traditional color cues.
6. Address Book
I use it a lot and while the look of the new version is nice, it tries to be too smart for me. I like the way the traditional AB looks with three columns and I dislike having to go back and forth from the groups area to individual cards. It's just not comfortable for me.
Most of the time when I use Spotlight to search for something, I like to know where that file is located. In the Snow Leo Spotlight, a tooltip appears when you mouse over an item that shows the path. This is much easier for me than having to do a cmd-click to open the file's enclosing folder. I don't want to go to the folder, I just want to know where it is.
While I've been using Lion I noticed a number of apps that don't work under Lion, though developers assure me that there will be Lion versions soon after it's announced. Having gone through the same process with other OS X releases, I have a lot of confidence in the development community
In closing I do want to mention that there are some very attractive new features but they aren't compelling enough for me to buy and install the FCS copy. There are other new features that will make a lot of people pretty excited but are total yawners for me (such as full-screen apps). I am certain that Lion will be very appealing to new users and old, just not to this ancient one.