Several factors to consider when getting a drawing tablet include pressure sensitivity, size, drivers, features and of course the price.
Having a stylus with good pressure sensitivity is a must. Tablets nowadays go up to 2048 levels of pressure sensitivity. The Wacom Intuos 3 that I'm still using has 1024 levels and that is quite adequate. So definitely get at least 1024 levels or above.
Another feature good to have is the tilt sensitivity. This feature depends on the support of graphics software. This appeals more to those who use brushes that can make use of the tilt sensitivity, e.g. airbrush.
Most drawing tablets are designed for widescreen monitors nowadays. Common sizes are 10 by 6 and 8 by 5 inches where the ratio is around 1.67:1 (a widescreen 16:9 monitor has a ratio of 1.78). Just be careful not to get the wrong proportion so that you can maximise the use of the whole of the drawing area.
Other features to consider but non are deal breakers (at least to me) are things like having an eraser on the back of the pen, customisable shortcut buttons, touch function and wireless capability. I've always preferred using the keyboard with the pen so the lack of eraser and shortcut buttons don't bother me.
To determine what size to get, you should take into account the size of your monitor. 8 by 5 inch is a good size to get, and if you have more budget the 10 by 6 inch. Personally I would go for a 10 by 6 inch because I use a 27-inch monitor at home. The higher the tablet to screen ratio, the bigger the tablet, and the more expensive it will be. Drawing on the tablet is intuitive but will need some time getting used to, and some training of your muscle memory. With a larger surface area, you have more room to manoeuvre for adding details. Don't forget to take into account how much free space your table has also.
The last important factor is the driver support. The drivers must be able to support the OS you're using, Windows or Mac. Also check if your drawing software is supported. Most tablets work well with Photoshop. But for lesser known software like GIMP or PaintTool Sai certain functions, such as shortcut buttons or pressure sensitivity may not work.
Important note on drivers: Windows users are recommended to install the drivers before plugging in the tablet. Otherwise, Windows will install their own drivers, and the pressure sensitivity will not work, and sometimes the Windows driver will be difficult to uninstall. This applies to tablets for all brands.
Alright, let's look at the drawing tablets that are available out there now.
COMPARISON OF DIFFERENT DRAWING TABLETS