physical aspects of siddurim
My criteria for a siddur are, roughly in order:
- content (but that's a separate discussion)
- legibility (crispness matters at least as much as font size)
- holdable in one hand
- paper not too thin (the "old Plaut chumash" problem)
Gates of Prayer (both blue and gray editions), Sim Shalom, and our home-grown Shabbat morning service in a three-ring binder all fulfill "holdabiltiy". The paperback Mishkan T'filah does; I'm a little concerned about a hardcover edition with the larger page size and all that extra content (almost 700 pages), but we'll see. I'll use the book anyway, as its content is miles ahead of GOP in my opinion, so this is just a question about how much hassle it will be.
Why is being able to hold it in one hand important? Because I want to have a hand free to turn pages and to hold stuff (like tzitzit during Kri'at Sh'ma, a lulav and etrog in their season, or a magnifying glass when a siddur violates the legibility goal), and I don't see well enough to just rest the siddur on the top of the pew in front of me when I need a free hand.
I don't care about outside factors like cover color/art beyond "would this embarrass me in a non-denominational minyan?". I don't particularly care about hardcover versus softcover, but the latter tend to get beat up as people fold pages back and that's not so good. But I don't feel that I'm having a lesser prayer experience if my siddur is softbound, spiral-bound, or in a binder. First and foremost for me are content and usability.

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They are also doing editions of MT with and without the transliteration; we're buying some of the latter specifically for use by students. What I haven't heard is whether the non-translit book will be smaller or laid out differently, or if there'll just be gobs of white space.
Oh, another thing I don't care for in siddurim -- excess white space for the sake of "aesthetics" if it ends up costing too much weight. MT has some problems there, though not severe in the last draft I saw.