A couple questions:
- What programming language(s) are you familiar with and/or which ones would you like to use?
- How much development experience do you have?
- Are you looking for any specific set of features, such as scene management, collision detection, alpha blending, sprite rotation, etc.?
- Do you want access to the source? Do you want any particular license (eg. LGPL, BSD)?
- Are they any set of included tools that you would like or need, such as a map editor?
- Does the engine need to rely specifically on 2D techniques (ie blitting) for rendering? I understand that you want your game to be 2D, but emulating a 2D environment within a 3D engine is usually quite easy. Generally, this is how most 2D engines work, in order to take advantage of hardware and some special effects.
Here's a pretty good list of engines, and their capabilities:
http://gpwiki.org/index.php/Game_Engines
To that list I would probably also add SDL (Simple DirectMedia Layer) and PyGame. They are a bit lower level, but they do have some good functionally for just 2D games. Neither seem to have gotten much development attention lately, but both are well known, and a lot of people have some experience with at least one of them. Pyglet is another that is similar, but seems to have a nicer feature set. Its also quite a bit newer, so I don't believe it has a large community yet.
I've heard pretty good things about both HGE and the Popcap Framework, but I've only taken a cursory look at them. I've used OGRE a fair amount, but it might be a bit heavy-weight for what you are looking for. I also took a good look at Panda3D, but again, it may be too much.
Depending on what you want to do, you may may also want to look at FRUA (Forgotten Realms Unlimited Adventures) and Dungeon Craft. They're definitely more on the tool end of the spectrum, and allow you to develop games similar to the old SSI Gold Box games.
For the most part, you're probably going to have to bite the bullet, and try out some of the engines yourself. Everybody has a different style and need, so picking an engine will mostly be about how you feel about each of them, where you feel you can compromise, and what you can put up with.