Tablets, Netbooks, & iPADs.. Oh My!
Okay, so I'll try to provide some insight into the questions you asked:
Q: Can you explain the principal differences between a Netbook and tablet?
A: A netbook typically runs an operating system designed for computers like Windows or Linux while the newer tablets run a mobile OS like Apple's iOS and Android Honeycomb. Also netbooks have built in keyboards; most tablets do not. If you'd like to have the keyboard option the ASUS Transformer offers a detachable keyboard dock that turns their tablet into an Android netbook.
Q: Can a tablet be used as a laptop replacement--why and why not?
A: I would have to say for the most part that a tablet cannot be used as a laptop replacement. First the operating system is different. Second the processing power of tablets aren't quite as substantial. I guess it all depends on what you plan on using the tablet for. However, due to the difference in the mobile operating system versus the operating system found on a standard laptop (Windows) you won't be able to run the exact same programs meant for Windows on a tablet.
Q: With so many tablets out, what separates the good- from the bad-quality ones?
A: A few suggestions here, 1) go with a brand name you recognize, 2) read reviews about the devices so you can learn from the experience of others, 3) see if you local technology store like Best Buy has any of the devices on display so you can get a better feel.
Q: Are Android tablets created equally?
A: Most of the newer Android tablets are pretty good, some of the first Android tablets that came out early on are not so good. So I would say they are not all created equally. Every tablet will have it's pro and cons, you just have to weight them against what you expect to get out of it and use it for. Again reading online reviews doesn't hurt.
Q: What are the major differences between Android and iPads--advantages, disadvantages?
A: iPADs are the only tablet Apple makes so your only decision is if you want an iPAD 1 or 2, I've read that the iPAD 3 is coming soon; probably early next year. iPAD (as far as I know) do not allow multitasking which means you can only use on app at a time on the device. iPADs do not support Flash media which is used on a moderately high amount of websites. The good thing about iPADs is that you know they are of good quality because they're made by Apple and their are tons of accessories made for iPADs. Apple offers a lot more apps for their iPAD than are currently available for Android tablets.
Android tablets are made by many different companies; Samsung, Sony, ASUS, Dell, and the list goes on. This means you have a lot more decisions to make when choosing an Android tablet. Each of the different brands have their own unique qualities and features so you'd have to research what features you think you might use. Android tablets, right now, do not have a whole lot of apps available on the Android market. The number of apps is slowly growing, however, with the latest version of Android Honeycomb you should be able to use most of the app designed for smartphone on your Android tablet. So make sure the Android tablet you get comes with Honeycomb which is Android OS version 3.x. Android tablets do allow multitasking and also support Flash media so you can see the Flash content on websites.
Q: Can I use a Tablet for word processing or Excel spread sheets?
A: Yes, there are several different word processing apps available for both Android tablets and iPADs, however, most of them are pay apps. Note that these apps may not always support every office document type especially if they are made on a desktop computer. For the most part though, they should work just fine.
Q: I think iPads are the ones that cannot display Flash, is that a major issue?
A: iPADs do not support Flash, I personally don't consider this a major issue, it primarily has to deal with viewing websites that include Flash content but not all sites use Flash. This is really just a perk for Android users. There are some games for Android that require flash. One minor down-side to Flash is that it often is found to have security holes, but Adobe often releases updates to fix these. Flash is a choice with Android, you need to have the Flash app installed on the device in order to get the benefits of this feature, but you can choose to not install it.
Q: What are the specific things I should be looking for when choosing the right tablet for me?
A: What you plan to use it for, how you plan to use it, how you expect it to function. If you want something that's easy to use I'd recommend an iPAD, they're straight forward, and they just work. If you need something with more advanced features and don't mind tinkering a bit more than Android is the way to go. Android Honeycomb is a bit more complicated to learn but offers more bells and whistles. The downside to Android is that you may on occasion run into glitches or bugs such as apps crashing or the tablet restarting itself. These issue are typically rare but they do occur. It also depends on what features you want. The iPAD2 doesn't offer a whole lot of hardware features other than the cameras and connectors whereas each different brand of Android tablet offers many different options and features including a microSD port, hdmi out so you can connect it to a TV, and a few offer usb ports for use with thumb drives. If you want something that's similar to the iPAD but runs Android I'd recommend the Samsung Galaxy Tab 10.1. The only other thing to look as is price, some Android tablets are quite pricey while others are fairly priced. Depending on what you plan to use the tablet for you may also want to look at memory options; many have 16GB models, and 32GB models. This affects how many apps you can have installed as well as how many documents, files, photos, and videos you can have installed on the device at any given time. For the most part I don't think the average user would need more than 16GB of storage but again that's a personal decision.
Hopefully that helps, if I missed anything I imagine someone else probably posted it. Happy hunting!