These are some really general and broad questions. My best advice would be to search google for pro's and con's because you get a lot of different perspectives.
One of the biggest note-worthy changes for me is the Configuration Profile Manager. If Apple can really nail this down it could change the way sysadmins manage policies forever.
Biggest con I have is that I feel Lion "Server" is really not a "Server" product as much as it is a "Service Add On Pack".
As best as I can tell, it's no different, in terms of installing server OS vs. installing client OS, than Snow Leopard Server was to Snow Leopard, by which I mean to say that the only thing separating an installation of Snow Leopard client from an intallation of Snow Leopard Server was an additional "Server Essentials" package that is installed as part of the installation. Similarly, you have the same thing with Lion except this time, you install the Server package separately and the installer downloads the package data upon installation.
Though if you're talking about in terms of functionality that is sad to hear. Again, I've only fiddled around with it for an hour, but it did seem like the basic essential services (Open Directory, Mail, iChat, iCal, Address Book, Software Update, NetBoot) were still there. Though again, I am a newbie even to Snow Leoapard Server and am only the ACTC 10.7 certification (and the prep material) shy of the same on Lion Server, so I really don't know what I'm talking about.
I'm an ACSA, and Lion Server is sucky, to say the least. It is still EXTREMELY buggy, and Apple does not seem to be resolving these problems in a timely manner... but onto the feature changes.
The normal preinstalled server tools are replaced with a Server app. This is literally the same thing as Server Preferences on SL Server, but more dumbed down and buggy. Server Admin, Workgroup Manager, etc can all be installed, but this is the best part: most of the services aren't accessible!! You can't edit (off the top of my head) web, VPN, iChat, etc from here! You can only do it from the Server app or from Terminal (which is not feasible for more complex configurations in web). And these settings are not shown in the Server app so they cannot be edited from the GUI...
Apple has also dropped a lot of support for Windows, MySQL, Quicktime Streaming Server, etc. This justifies the 90% price reduction.
Ok, now about the only good thing is that it has really cool iOS device support and push notification support. You can install configurations profiles (for managing mail, calendar settings, etc) on iOS devices off of your website and always keep them up to date through the easy to use push mechanism built in. I really like this, but I don't have any use for it and it does not make up at all for all of the features lost.
I would not recommend upgrading to Lion. No offense, but I think you are wasting your time by becoming certified in Lion. I will not be and, if anything, become certified in Windows Server. It's unfortunate because while OS X is still great, Apple is ruining it for slightly more advanced users (never mind us!) and they are helping Windows to win over more people. I WANT to use a Mac, but Apple is making it a bad experience that does not fulfill my needs. Their server is inferior (due to them, they had a decent server) and not many people will continue to use it in the future. Most of the same services can be provided by another OS that is not dumbed-down, impossible to edit, and buggy.
I did notice the drop of QuickTime Streaming Server, didn't notice the drop in MySQL support and am kind of shocked that they'd do something that stupid, nor do I notice the drop in support for Windows services. What specific Windows services are no longer in Lion Server where they were in Snow Leopard Server? Surely, they must still support connecting to an Active Directory as well as serving up SMB shares. Bizarre that they neutered the Server Admin app given that, as I recall, for things non-pertinent to the domain, it was THE place to go, and forcing admins to edit from the terminal where there had previously been GUI functionality that was just as powerful. The iOS stuff seems very cool, though with the new "Apple Configurator" app that they now have for both Lion (client) and Lion Server on the Mac App Store, I wonder how much you need Lion Server for that kind of stuff.
As for my desire to study the "OS X Lion Server Essentials" stuff for my ACTC 10.7 certification, I'm well aware that adoption of Lion Server will be small if not nil. People will either stick with Snow Leopard Server or migrate to Windows Small Business Server 2011 or something similar. Really, if I thought that the ACSP 10.7 certification would be enough to give me a leg up on all of the Mac IT gigs out there, I'd stop right there. Alas, I've found that there are a ton of employers out there that don't care what the Apple Certiications are, but just zero in on the title and how many of them you have.
For this reason, I also spent the two hours preparing and taking the Mac Integration Basics 10.7 exam for my Apple Certified Associate - Mac Integration 10.7 certification. It is incredibly stupid and really only measures that I have a high school reading level, have used Lion for more than five seconds, and know how to take an open-note/book exam, but nevertheless, it's an additional certification with a title that someone who doesn't know what they mean will appreciate. I was going to go for the ACSA 10.6 certification for the same reasons, but I incorrectly figured that one couldn't earn the 10.6 certs as soon as the 10.7 ones were out, only to find out two months ago that such was incorrect. I don't have time to do all three elective exams, nor do I really want to, so I'm going to just take the Mac OS X Deployment 10.6 exam as that is the only elective topic/exam/ACS-cert that interests me. Though with the ACS-Deployment 10.6 cert, I'm not preparing for it because I think that cert will get me a leg up, but rather because it has been the most fun I've had academically in years (and sadly I include the work for my BA in that as well) and I'd like to see it through and learn more about it out of genuine interest rather than professional advancement.
Back on topic, my aim in getting the ACTC 10.7 is more to stand out from those that only have the ACSP 10.7, and in doing so, I'd rather learn about Lion Server from the standpoint of having come from Snow Leopard Server than coming to it from scratch. Otherwise, I agree; a Windows Server certification would mean tons more, but I hate working with Windows and would rather have a job working with Lion-based (or any other version of OS X for that matter) clients, even if it means I'm getting a Lion Server certification in order to (stand out to nimrods that don't know anything about what the certifications actually certify in order to) do so.
