The UK claims that the use of enhanced cooperation here is fundamentally against the EU treaties as it imposes costs on those outside the FTT-zone. If the ECJ rules against the UK, it could become a wide-ranging precedent with three key effects:
Allowing for the broader use of enhanced cooperation (even with extraterritorial impacts) including for eurozone integration
Making it more difficult for the UK to employ a veto over further EU integration it's not part of
Undermining trust in the ECJ as a fair, impartial arbiter and guardian of the single market
However, it's important not to be too alarmist about this. While the ruling looks unlikely to go in the UK's favour (but it still could) it seems more likely to be dismissed on grounds of the UK's challenge being premature (given that the proposal is yet to be finalised) rather than being outright wrong. So the UK will have another shot at challenging the final decision.
Unless a labour government are in power when the time comes.
http://openeuropeblog.blogspot.co.uk/2014/04/judgement-day-for-eu-ftt-will-ecj-rule.html