Most important: Is the sound engine updated? Until Logic 9 it was stuck in the 80's and sounded like crap compared to other modern DAWs. You could even hear artefacts in fade-outs or effect plug-ins.
I don't care much about a new UI as long as they make Logic sound better.
What's this "Until Logic 9" bit? It sounds as if your beef is with Logic 7 or 8, not 9. So then, what is wrong with Logic 9's sound engine? You list no reason what-so-ever to update it.
I found nothing objectionable about the results with my last album using Logic9. In fact, the sound quality is amazing for a home studio, easily rivaling the better rock albums I own. I'm not sure what they would need to "upgrade" on it.
My biggest complaints are about the low quality default guitar stomp box mix presets and the quality of their stomp boxes in general (i.e. it took me a good long while to get a lead guitar sound that reminded me of Pink Floyd, namely the sound of the lead guitar in Comfortably Numb). Most of the defaults were out and out USELESS/WORTHLESS. On the other hand, I remember most of the presets on my first Digisound outboard box being total crap as well (about 5 usable out of 200). Even so, I was able to get guitar sounds I liked for the album out of it without having to use external gear that would likely increase the noise, etc. on the album.
Similarly, many of the soft-synth presets in Logic 9 are pretty useless/crappy too without a lot of finessing or experimentation with additional effects. I ended up using a lot of external Roland synth sounds on the album (e.g. my Roland saxophone sounds were night and day better than the ones Apple included in Logic and that's on a Roland I bought back around 2001! The difference is that the Rolands sound nearly real when played within certain constraints while Apple's sound bland. I ended up buying a lot of 3rd party sounds (e.g. I got some Great Moog sounds for Genesis or Tron Soundtrack type moods in the background). A lot of Logic's synth engines CAN make interesting sounds, but it requires a lot of fiddling with the virtual knobs and sliders to get there. I spent too many hours fiddling, IMO when I just wanted to work on a song, but none of the synths fit with the mood of the song.
In short, I'm pleased to hear Logic X has a lot of new guitar and synthesizer sounds included with it. The idea of an all-in-one processing center where I can plug a keyboard controller or electric guitar into a firewire box and let the computer handle all the processing effects or sound patches has a lot of appeal. Unfortunately, Logic 9 just didn't have ENOUGH sounds to please me for a single album and I'm just one person (and my album is more guitar orientated). I ended up using a lot more traditional piano than I imagined for this reason. Sometimes the real instruments are the best.
I wouldn't mind some easier features (or techniques) to cut up songs in the middle to add a bridge or other section. I did this a lot in the initial song shaping, etc., but it gets increasingly a PITA when you have 20 tracks to cut evenly and splice back together adding parts to a more finished product. I got more proficient at it as time went on, but I still felt like either there was a better way to do it I didn't know about or Logic could be a bit smarter about picking the ideal cut points and/or tying loose ends together. I mean ultimately, you want a new section to tie-in and out as if it was there all along and if that was considerably easier, I think I'd be writing a lot more songs just for fun. Editing can be a real pain when you just want to fiddle with the song and hear how something added or removed sounds (tying the loose threads together).
Otherwise, the new iPad controller sounds awesome as well. I spent a lot of time moving around to hear the final mixes on various audio systems in the house to make sure it sounded good on all of them and being able to tweak levels and other things from a WiFi iPad anywhere in the house would be simply awesome (I had to bounce mixes to test in the home theater, den, etc.) The only question is whether Logic could easily send the output to an AppleTV or the like in another location directly or whether I'd have to use something like AirFoil to do it. It's easy on my latest Mac Mini Server to send audio and video to AppleTV directly, but I don't think my 2008 Macbook Pro I use Logic9 on has that capability even with Mountain Lion installed on it.